github: Test a known-good version of ffmpeg

The latest version in git might have some tests failing occasionally
(like it does currently).

.github/workflows/build.yml

@@ -304,6 +304,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: ffmpeg/ffmpeg
+          ref: n5.1.2
           path: ffmpeg
       - name: Build ffmpeg
         run: |
@@ -358,6 +359,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: ffmpeg/ffmpeg
+          ref: n5.1.2
           path: ffmpeg
       - name: Build & test ffmpeg
         run: |