Add alls-green job to CI workflow

Author: cclauss

.github/workflows/ci.yml

@@ -86,3 +86,16 @@ jobs:
         env:
           # Should match 'name:' up above
           JOB_NAME: 'macOS (${{ matrix.python }})'
+
+  alls-green:
+    if: always()
+    needs:
+    - Windows
+    - Ubuntu
+    - macOS
+    runs-on: ubuntu-latest
+    steps:
+    - name: Decide whether all jobs succeeded or not
+      uses: re-actors/alls-green@release/v1
+      with:
+        jobs: ${{ toJSON(needs) }}