fourier-debian: fix mpv Conflicts: + daedalus ffmpeg conflict

Two deterministic CI failures from run 115 (PR #34 wiring):

mpv-fourier-debian (task 566): dpkg-deb refuses `Conflicts: pkg1 |
pkg2` — Debian policy doesn't allow alternatives in Conflicts.
Split into separate entries.

daedalus-v4l2-debian (task 563): pacman -Syu pulled stock arch
ffmpeg which conflicts with the already-installed
ffmpeg-v4l2-request-fourier left behind by the earlier
mpv-fourier-aarch64 job (it configures [marfrit] + pre-installs
the fourier ffmpeg).  Drop 'ffmpeg' from the pacman line; mirror
the [marfrit]+pre-install step from mpv-fourier-debian.  Daedalus
only needs libavcodec/libavformat headers which the fourier
package supplies.

debian/mpv-fourier/build-deb.sh

@@ -109,7 +109,7 @@ Depends: ffmpeg-v4l2-request-fourier (>= 2:8.1+rfourier),
          libxkbcommon0, libxext6, libxpresent1,
          libxrandr2, libxss1, libxv1, libx11-6,
          zlib1g
-Conflicts: mpv, libmpv2 | libmpv1
+Conflicts: mpv, libmpv2, libmpv1
 Replaces: mpv, libmpv2, libmpv1
 Provides: mpv (= ${PKGVER}-${PKGREL}), libmpv.so.2
 Maintainer: Markus Fritsche <mfritsche@reauktion.de>