github: Update to building and testing ffmpeg 9.0.2 instead of 6.1.2

This also avoids the problem with building on Ubuntu 26.04 if
building docs isn't disabled. (We're not interested in building
the docs here anyway, so keep it disabled in any case.)

.github/workflows/build.yml

@@ -720,7 +720,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           repository: ffmpeg/ffmpeg
-          ref: n6.1.2
+          ref: n9.0.2
           path: ffmpeg
       - name: Build ffmpeg
         run: |
@@ -774,7 +774,7 @@ jobs:
         uses: actions/checkout@v6
         with:
           repository: ffmpeg/ffmpeg
-          ref: n6.1.2
+          ref: n9.0.2
           path: ffmpeg
       - name: Build & test ffmpeg
         run: |