PKGBUILD already renamed itself (pkgname=ffmpeg-v4l2-request-fourier,
replaces=(ffmpeg ffmpeg-v4l2-request-git)) but the containing
directory was never moved. This commit completes the rename to align
the path with the package identity and the rest of the -fourier
umbrella (libva, mpv, kwin, qt6-base, chromium, linux-*).
CI workflow path-trigger is wildcard (arch/**), unaffected. Step
names + cp source path updated to the new directory.