ci: convert daedalus-v4l2{,-dkms}-debian install steps to apt-get

PR #47 moved the daedalus-v4l2-debian + daedalus-v4l2-dkms-debian
jobs from runs-on: arch-aarch64 to runs-on: debian-aarch64, but
left the install-deps steps using `pacman -Syu` — which doesn't
exist on the Debian runner.  Both jobs were latent-broken; the
break only surfaces once a daedalus pkgver actually changes (the
rebuild guard skipped them in runs #133-134 since nothing about
daedalus moved between PR #47 and PR #48).

PR #49 bumped both daedalus packages to 0.1.0+r20+g3dd0eb0 (the
DAEMON-PPS H.264 SPS/PPS NAL synth landing) — so run #135's
daedalus-debian + daedalus-dkms-debian jobs actually executed and
hit the broken pacman step.  Result: instant failure on `pacman -Syu`.

Fix: replace the pacman invocations with apt-get equivalents.
For daedalus-v4l2-debian, drop the [marfrit] ffmpeg-v4l2-request-
fourier preinstall — Debian's stock libavcodec-dev / libavformat-
dev / libavutil-dev provide matching headers (both trixie ffmpeg
and the daedalus daemon's runtime dlopen target are libavcodec
61.x), and the daemon never link-binds against libav (Option γ —
dlopen at runtime), so any header set with the right struct
definitions works.

Verified end-to-end on higgs (Debian trixie aarch64, equivalent
to bohr): clone the source tarball, run build-deb.sh, produces
daedalus-v4l2_0.1.0+r20+g3dd0eb0-1_arm64.deb cleanly (10/10
ninja steps, daedalus_v4l2_daemon binary linked).

NOTE: ffmpeg-v4l2-request-debian (line ~907) and mpv-fourier-
debian (line ~1048) have the same pacman-on-Debian bug from
PR #47 but are still skipped because their pkgvers haven't moved.
Not fixing those in this PR to keep the change focused on
unblocking DAEMON-PPS verification — they'll need the same
treatment the next time they bump.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -1144,39 +1144,25 @@ jobs:
           echo "$result" >> "$GITHUB_OUTPUT"
           echo "decision: $result"
 
-      - name: install build-deps (sans ffmpeg — see [marfrit] step)
+      - name: install build-deps
         if: steps.skip-check.outputs.skip != '1'
         run: |
           set -e
           retry() { for i in 1 2 3; do "$@" && return 0; rc=$?; echo "retry $i (exit=$rc)" >&2; sleep $((i*5)); done; return 1; }
-          # Do NOT pull stock 'ffmpeg' here: the arch-aarch64 runner has
-          # ffmpeg-v4l2-request-fourier pre-installed from the mpv-aarch64
-          # job (configured via [marfrit]), and pacman -S ffmpeg would
-          # conflict on the libav* drop-in.  Daedalus build only needs
-          # libavcodec/libavformat headers, which the fourier package
-          # already supplies.  Keep cmake/ninja/pkgconf/libdrm here; the
-          # ffmpeg-dev equivalent comes via the next step.
-          retry pacman -Syu --noconfirm --needed \
-            dpkg openssh rsync curl base-devel git cmake ninja pkgconf \
-            libdrm
-
-      - name: ensure ffmpeg-v4l2-request-fourier installed (link-time ABI source)
-        if: steps.skip-check.outputs.skip != '1'
-        run: |
-          set -e
-          # Idempotent: pre-install the marfrit fourier ffmpeg so cmake
-          # finds libavcodec / libavformat / libavutil headers + .so's.
-          # Mirrors mpv-fourier-debian's [marfrit] step.
-          curl -sLo /tmp/marfrit.gpg https://packages.reauktion.de/marfrit.gpg
-          pacman-key --add /tmp/marfrit.gpg
-          pacman-key --lsign-key 92D5E96D8F63C75E4116AA1FF5C8C4603D0D250C
-          rm -f /tmp/marfrit.gpg
-          if ! grep -q '^\[marfrit\]' /etc/pacman.conf; then
-            printf '\n[marfrit]\nServer = https://packages.reauktion.de/arch/$arch\nSigLevel = Required\n' >> /etc/pacman.conf
-          fi
-          pacman -Sy --noconfirm
-          rm -f /var/cache/pacman/pkg/ffmpeg-v4l2-request-fourier-*-aarch64.pkg.tar.*
-          printf 'y\ny\ny\n' | pacman -S --needed marfrit/ffmpeg-v4l2-request-fourier
+          export DEBIAN_FRONTEND=noninteractive
+          retry apt-get update -qq
+          # libav*-dev provide the headers daedalus daemon dlopens at
+          # runtime — Debian's stock packages match the trixie ABI the
+          # daemon will encounter on Pi 5 hosts (both ship libavcodec
+          # 61.x).  The fourier ffmpeg fork isn't needed here; the
+          # daemon never link-binds against libav (Option γ — dlopen
+          # at runtime), so any header set with the right struct
+          # definitions works.
+          retry apt-get install -y --no-install-recommends \
+              build-essential cmake ninja-build pkg-config git \
+              libavcodec-dev libavformat-dev libavutil-dev libdrm-dev \
+              linux-libc-dev \
+              curl ca-certificates openssh-client rsync dpkg-dev
 
       - name: install hertz deploy ssh key
         if: steps.skip-check.outputs.skip != '1'
@@ -1238,7 +1224,10 @@ jobs:
         run: |
           set -e
           retry() { for i in 1 2 3; do "$@" && return 0; rc=$?; echo "retry $i (exit=$rc)" >&2; sleep $((i*5)); done; return 1; }
-          retry pacman -Syu --noconfirm --needed dpkg openssh rsync curl tar gzip
+          export DEBIAN_FRONTEND=noninteractive
+          retry apt-get update -qq
+          retry apt-get install -y --no-install-recommends \
+              dpkg-dev openssh-client rsync curl ca-certificates tar gzip
 
       - name: install hertz deploy ssh key
         if: steps.skip-check.outputs.skip != '1'