flatpak: manually install Zig 0.14.1

The SDK published on Flathub updated to Zig 0.15.1 which broke the
Flathub build in CI. So let's install it ourselves so that we can
control the version.

.github/workflows/test.yml

@@ -1013,32 +1013,31 @@ jobs:
       - name: Check Flatpak Zig Dependencies
         run: nix develop -c ./flatpak/build-support/check-zig-cache.sh
 
-  # Disabled until we update to Zig 0.15 or if we can pin this to Zig 0.14
-  # flatpak:
-  #   if: github.repository == 'ghostty-org/ghostty'
-  #   name: "Flatpak"
-  #   container:
-  #     image: ghcr.io/flathub-infra/flatpak-github-actions:gnome-47
-  #     options: --privileged
-  #   strategy:
-  #     fail-fast: false
-  #     matrix:
-  #       variant:
-  #         - arch: x86_64
-  #           runner: namespace-profile-ghostty-md
-  #         - arch: aarch64
-  #           runner: namespace-profile-ghostty-md-arm64
-  #   runs-on: ${{ matrix.variant.runner }}
-  #   needs: [flatpak-check-zig-cache, test]
-  #   steps:
-  #     - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-  #     - uses: flatpak/flatpak-github-actions/flatpak-builder@10a3c29f0162516f0f68006be14c92f34bd4fa6c # v6.5
-  #       with:
-  #         bundle: com.mitchellh.ghostty
-  #         manifest-path: flatpak/com.mitchellh.ghostty.yml
-  #         cache-key: flatpak-builder-${{ github.sha }}
-  #         arch: ${{ matrix.variant.arch }}
-  #         verbose: true
+  flatpak:
+    if: github.repository == 'ghostty-org/ghostty'
+    name: "Flatpak"
+    container:
+      image: ghcr.io/flathub-infra/flatpak-github-actions:gnome-47
+      options: --privileged
+    strategy:
+      fail-fast: false
+      matrix:
+        variant:
+          - arch: x86_64
+            runner: namespace-profile-ghostty-md
+          - arch: aarch64
+            runner: namespace-profile-ghostty-md-arm64
+    runs-on: ${{ matrix.variant.runner }}
+    needs: [flatpak-check-zig-cache, test]
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      - uses: flatpak/flatpak-github-actions/flatpak-builder@10a3c29f0162516f0f68006be14c92f34bd4fa6c # v6.5
+        with:
+          bundle: com.mitchellh.ghostty
+          manifest-path: flatpak/com.mitchellh.ghostty.yml
+          cache-key: flatpak-builder-${{ github.sha }}
+          arch: ${{ matrix.variant.arch }}
+          verbose: true
 
   valgrind:
     if: github.repository == 'ghostty-org/ghostty'