[CI] Move check_labels to github actions (#8099)

* move lib.sh to common dir

* make check-labels a github action workflow

substrate/.github/workflows/check-labels.yml

@@ -0,0 +1,16 @@
+name: Check labels
+
+on:
+  pull_request:
+    types: [labeled, opened, synchronize, unlabeled]
+
+jobs:
+  check-labels:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check labels
+        run: bash ${{ github.workspace }}/.maintain/github/check_labels.sh
+        env:
+          GITHUB_PR: ${{ github.event.pull_request.number }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}