github: Bump to building ffmpeg 6.1.2 rather than 6.0

This fixes building on Ubuntu 24.04 (the one currently used by
the github actions ubuntu-latest tag), by including a fix for texinfo
7.0.

Not bumping to ffmpeg 7.x, as some tests there can be flaky at times.

.github/workflows/build.yml

@@ -605,7 +605,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ffmpeg/ffmpeg
-          ref: n6.0
+          ref: n6.1.2
           path: ffmpeg
       - name: Build ffmpeg
         run: |
@@ -659,7 +659,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ffmpeg/ffmpeg
-          ref: n6.0
+          ref: n6.1.2
           path: ffmpeg
       - name: Build & test ffmpeg
         run: |