ci(flatpak): reuse builder cache and skip aarch64 on PRs (#535)

Using github.sha in the cache key forced a full rebuild on every commit.
Hash packaging/flatpak instead so .flatpak-builder can restore across
runs, and only rebuild modules that actually changed.

PRs build x86_64 only; main and workflow_dispatch still cover both arches.
Drop the extra upload-artifact step in favor of the action's own upload.

.github/workflows/flatpak.yml

@@ -24,6 +24,8 @@ concurrency:
 jobs:
   build:
     name: Build Flatpak (${{ matrix.arch }})
+    # PRs: x86_64 only. main / workflow_dispatch: both arches.
+    if: matrix.arch == 'x86_64' || github.event_name != 'pull_request'
     runs-on: ${{ matrix.runner }}
     strategy:
       fail-fast: false
@@ -45,15 +47,12 @@ jobs:
         with:
           bundle: vocalinux-${{ matrix.arch }}.flatpak
           manifest-path: packaging/flatpak/com.vocalinux.Vocalinux.yml
-          cache-key: flatpak-${{ matrix.arch }}-${{ github.sha }}
+          # Stable key so .flatpak-builder cache restores across commits.
+          # github.sha in the key forced a full rebuild every run.
+          # Arch is appended automatically by the action.
+          cache-key: flatpak-builder-${{ hashFiles('packaging/flatpak/**') }}
           arch: ${{ matrix.arch }}
+          artifact-name: vocalinux-flatpak-${{ matrix.arch }}
           # Flathub mirrors screenshots at submission time; mirror locally so the
           # builddir linter does not flag the external screenshot URL.
           mirror-screenshots-url: https://dl.flathub.org/media
-
-      - name: Upload bundle
-        uses: actions/upload-artifact@v4
-        with:
-          name: vocalinux-flatpak-${{ matrix.arch }}
-          path: vocalinux-${{ matrix.arch }}.flatpak
-          if-no-files-found: error