Add PR prefix to label

.github/workflows/process-pull-request.yml

@@ -12,4 +12,4 @@ jobs:
             -   uses: actions-ecosystem/action-add-labels@v1
                 if: ${{ github.repository_owner != 'Ultimaker' }}
                 with:
-                    labels: 'Community Contribution :crown:'
+                    labels: 'PR: Community Contribution :crown:'