ci: Pin macos to macos-12 for now

macos-latest now runs on Apple Silicon, which fails one test
and probably still needs extra work for the packaging to work.
Until that's figured out, run on macos-12.

.github/workflows/ci.yml

@@ -64,13 +64,13 @@ jobs:
               -Dvapoursynth=enabled
           - {
             name: macOS Debug,
-            os: macos-latest,
+            os: macos-12,
             buildtype: debugoptimized,
             args: -Ddefault_library=static -Dbuild_osx_bundle=true -Dlocal_boost=true --force-fallback-for=ffms2
           }
           - {
             name: macOS Release,
-            os: macos-latest,
+            os: macos-12,
             buildtype: release,
             args: -Ddefault_library=static -Dbuild_osx_bundle=true -Dlocal_boost=true --force-fallback-for=ffms2
           }