mpv-fourier pkgrel=8 + workflow: clear corrupt pacman cache before reinstall

Run #73 hit:
  warning: ffmpeg-v4l2-request-fourier-... is up to date -- reinstalling
  File /var/cache/pacman/pkg/ffmpeg-v4l2-request-fourier-...pkg.tar.xz
  is corrupted (invalid or corrupted package (checksum)).

The corrupt cached file is left over from the ccache misadventure
that interrupted earlier builds mid-download. Pacman doesn't auto-
redownload corrupt cached files in this code path. Add a targeted rm
of cached ffmpeg-v4l2-request-fourier-*.pkg.tar.* before the install
so the next pacman -S downloads fresh.

Yak shave count for mpv-fourier-aarch64 to actually run: 6
(pkgrel→4 to outrank stock, [marfrit] config, ffmpeg-v4l2-request
explicit dep, conflict-prompt acceptance, pipefail-friendly printf,
cache rm).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -649,6 +649,10 @@ jobs:
             printf '\n[marfrit]\nServer = https://packages.reauktion.de/arch/$arch\nSigLevel = Required\n' >> /etc/pacman.conf
           fi
           pacman -Sy --noconfirm
+          # Drop any cached ffmpeg-v4l2-request-fourier that may be corrupt
+          # from a prior interrupted build (the ccache misadventure left
+          # one such file behind).
+          rm -f /var/cache/pacman/pkg/ffmpeg-v4l2-request-fourier-*-aarch64.pkg.tar.*
           # Stock arch ffmpeg may already be installed from a prior fermi job;
           # pacman -S --noconfirm defaults the [y/N] conflict prompt to N.
           # Use printf (finite stream, exits 0 cleanly) — `yes y | ...` would