ci: try multiarch flatpak builder

.github/workflows/flatpak.yml

@@ -9,14 +9,28 @@ jobs:
     container:
       image: bilelmoussaoui/flatpak-github-actions:gnome-43
       options: --privileged
+    strategy:
+      fail-fast: false
+      matrix:
+        arch: [x86_64, aarch64]
     steps:
     - uses: actions/checkout@v3
       with:
         submodules: recursive
         fetch-depth: 0
+    # Docker is required by the docker/setup-qemu-action which enables emulation
+    - name: Install deps
+      run: |
+        dnf -y install docker
+    - name: Set up QEMU
+      id: qemu
+      uses: docker/setup-qemu-action@v1
+      with:
+        platforms: arm64
     - uses: flatpak/flatpak-github-actions/flatpak-builder@v4
       with:
         bundle: ghostty.flatpak
         manifest-path: com.mitchellh.ghostty.yml
         branch: tip
-        cache-key: flatpak-builder-${{ github.sha }}-v1
+        cache-key: flatpak-builder-${{ matrix.arch }}-${{ github.sha }}-v1
+        arch: ${{ matrix.arch }}