build(deps): bump flatpak/flatpak-github-actions from 5 to 6

Bumps [flatpak/flatpak-github-actions](https://github.com/flatpak/flatpak-github-actions) from 5 to 6.
- [Release notes](https://github.com/flatpak/flatpak-github-actions/releases)
- [Commits](https://github.com/flatpak/flatpak-github-actions/compare/v5...v6)

---
updated-dependencies:
- dependency-name: flatpak/flatpak-github-actions
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build.yml

@@ -27,7 +27,7 @@ jobs:
         uses: docker/setup-qemu-action@v2.1.0
         with:
           platforms: arm64
-      - uses: flatpak/flatpak-github-actions/flatpak-builder@v5
+      - uses: flatpak/flatpak-github-actions/flatpak-builder@v6
         with:
           repository-name: gnome-nightly
           bundle: bavarder.flatpak