CI: Build Flatpak into different branches when publishing

When publishing to Flathub, we want to publish the 'beta'
branch to the 'beta' repository, and the 'stable' branch
to the 'stable' repository. However, we currently publish
the 'master' branch for both.

Build the Flatpak manifest with different branches for each
repository.

.github/workflows/flatpak.yml

@@ -65,6 +65,7 @@ jobs:
     env:
       FLATPAK_BUILD_PATH: flatpak_app/files/share
       FLATHUB_REPOSITORY: beta
+      FLATHUB_BRANCH: beta
       FLATHUB_TOKEN: ${{ secrets.FLATHUB_BETA_TOKEN }}
     container:
       image: bilelmoussaoui/flatpak-github-actions:kde-5.15-21.08
@@ -82,6 +83,7 @@ jobs:
           manifest-path: CI/flatpak/com.obsproject.Studio.json
           cache-key: flatpak-builder-${{ github.sha }}
           mirror-screenshots-url: https://dl.flathub.org/repo/screenshots
+          branch: ${{ env.FLATHUB_BRANCH }}
 
       - name: Validate AppStream
         shell: bash
@@ -116,6 +118,7 @@ jobs:
     env:
       FLATPAK_BUILD_PATH: flatpak_app/files/share
       FLATHUB_REPOSITORY: stable
+      FLATHUB_BRANCH: stable
       FLATHUB_TOKEN: ${{ secrets.FLATHUB_TOKEN }}
     container:
       image: bilelmoussaoui/flatpak-github-actions:kde-5.15-21.08
@@ -133,6 +136,7 @@ jobs:
           manifest-path: CI/flatpak/com.obsproject.Studio.json
           cache-key: flatpak-builder-${{ github.sha }}
           mirror-screenshots-url: https://dl.flathub.org/repo/screenshots
+          branch: ${{ env.FLATHUB_BRANCH }}
 
       - name: Validate AppStream
         shell: bash