Add label about workflow to new contributor PRs

This modifies the new contributor bot workflow to add a "status: needs workflow approval" label. This is because new contributors PRs cannot be run through CI without approval from someone with commit rights, but it is not immediately clear which PRs are from new contributors. I'm using "status: needs workflow approval" over "new contributor" because it is tied to a specific actionable task.

Author: story645

.github/workflows/pr_welcome.yml

@@ -9,6 +9,7 @@ jobs:
     steps:
     - uses: actions/first-interaction@v1
       with:
+        add-labels: "status: needs workflow approval"
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         pr-message: >+
           Thank you for opening your first PR into Matplotlib!