github: Run tests with the ffmpeg 6.0 release instead of 5.1.2

Author: mstorsjo

.github/workflows/build.yml

@@ -432,7 +432,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: ffmpeg/ffmpeg
-          ref: n5.1.2
+          ref: n6.0
           path: ffmpeg
       - name: Build ffmpeg
         run: |
@@ -487,7 +487,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: ffmpeg/ffmpeg
-          ref: n5.1.2
+          ref: n6.0
           path: ffmpeg
       - name: Build & test ffmpeg
         run: |