ci: clean artifacts, move flatpak to release tip

.github/workflows/clean-artifacts.yml

@@ -0,0 +1,17 @@
+name: Clean Artifacts
+on:
+  schedule:
+    # Every 6 hours
+    - cron: '0 */6 * * *'
+  workflow_dispatch:
+jobs:
+  remove-old-artifacts:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+    - name: Remove old artifacts
+      uses: c-hive/gha-remove-artifacts@v1
+      with:
+        age: '1 week'
+        skip-tags: true
+        skip-recent: 5