github: try to enable building flatpak bundle

.github/workflows/c-cpp.yml

@@ -62,3 +62,28 @@ jobs:
       with:
         name: artifact-${{ matrix.targetos }}-${{ matrix.targetarch }}
         path: artifacts/*
+  flatpak:
+    name: "Flatpak"
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - app: su.xash.Engine.Compat.i386
+    container:
+      image: bilelmoussaoui/flatpak-github-actions:freedesktop-22.08
+      options: --privileged
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      UPLOADTOOL_ISPRERELEASE: true
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v3
+      with:
+        submodules: recursive
+    - name: Build flatpak (Compat.i386)
+      uses: FWGS/flatpak-github-actions/flatpak-builder@v5
+      with:
+        bundle: ${{ matrix.app }}.flatpak
+        manifest-path: scripts/flatpak/${{ matrix.app }}.yml
+    - name: Upload engine (prereleases)
+      run: bash scripts/continious_upload.sh ${{ matrix.app }}.flatpak