ci: don't deploy to preview for PRs that originate from dev

Author: stdavis

.github/workflows/pull_request.yml

@@ -68,7 +68,7 @@ jobs:
     name: Firebase preview
     runs-on: ubuntu-latest
     needs: [test-unit-ui, test-unit-python]
-    if: ${{ github.event.sender.type == 'User' }}
+    if: ${{ github.event.sender.type == 'User' && github.head_ref != 'dev' }}
     environment:
       name: dev