actions: nightly: Manually install SDK extensions via workflow

The github actions Flatpak action only allows to specify single source
for installing missing dependencies. This makes impossible to build
some things from "gnome-nightly" while others from "flathub" repo.

Work around this limitation by manually installing missing SDK
extensions from Flathub prior to using this git action.

.github/workflows/flatpak-nightly.yml

@@ -26,8 +26,12 @@ jobs:
       uses: docker/setup-qemu-action@v1
       with:
         platforms: arm64
+    - name: Prepare Runtime
+      run: |
+        flatpak --system install -y --noninteractive flathub org.freedesktop.Sdk.Extension.rust-nightly/${{ matrix.arch }}/21.08
+        flatpak --system install -y --noninteractive flathub org.freedesktop.Sdk.Extension.llvm13/${{ matrix.arch }}/21.08
     - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v4
-      name: "Build"
+      name: Build
       with:
         bundle: com.github.rafostar.Clapper.flatpak
         manifest-path: pkgs/flatpak/com.github.rafostar.Clapper-nightly.json