Actions: bump flatpak to 6.4 (#1540)

Author: danirabbit

.github/workflows/ci.yml

@@ -55,7 +55,7 @@ jobs:
           platforms: arm64
 
       - name: Build
-        uses: flatpak/flatpak-github-actions/flatpak-builder@v6
+        uses: flatpak/flatpak-github-actions/flatpak-builder@v6.4
         with:
           bundle: code.flatpak
           manifest-path: io.elementary.code.yml