[maos] Unlink X11 libraries from homebrew

Even though we disable XCB support in FFmpeg, somehow the X11 libraries
still end up linked into the build, resulting in errors:

    delocate.libsana.DelocationError: Library dependencies do not satisfy target MacOS version 11.0:
      [..]/libXdmcp.6.dylib has a minimum target of 14.0
      [..]/libXau.6.0.0.dylib has a minimum target of 14.0
      [..]/libX11.6.dylib has a minimum target of 14.0
      [..]/libxcb.1.1.0.dylib has a minimum target of 14.0

.github/workflows/build-ffmpeg.yml

@@ -35,7 +35,7 @@ jobs:
         run: |
           brew update
           brew install pkg-config
-          brew unlink gettext libidn2 libpng libtiff libunistring little-cms2 unbound
+          brew unlink gettext libidn2 libpng libtiff libunistring libx11 libxau libxcb libxdmcp little-cms2 unbound
       - uses: msys2/setup-msys2@v2
         if: matrix.os == 'windows-latest'
         with: