chore(deps): Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/build-ffmpeg.yaml

@@ -71,14 +71,14 @@ jobs:
 
     steps:
       - name: Checkout ${{ needs.check.outputs.helper-git-owner }}/ffmpeg-windows-build-helpers
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: ${{ needs.check.outputs.helper-git-owner }}/ffmpeg-windows-build-helpers
           ref: ${{ needs.check.outputs.helper-git-sha }}
           persist-credentials: false
 
       - name: Checkout FFmpeg
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           repository: FFmpeg/FFmpeg
           ref: ${{ needs.check.outputs.git-sha }}