[ci] add an allgood job for easy [required] builds (#3351)

* add an allgood job for easy [required] builds

see https://github.community/t/is-it-possible-to-require-all-github-actions-tasks-to-pass-without-enumerating-them/117957/4?u=graingert



* Update .github/workflows/main.yml

* Update .github/workflows/main.yml
Co-authored-by: Thomas Grainger <tagrain@gmail.com>
Co-authored-by: James Lamb <jaylamb20@gmail.com>

.github/workflows/main.yml

@@ -160,3 +160,10 @@ jobs:
           $env:TASK = "${{ matrix.task }}"
           conda init powershell
           & "$env:GITHUB_WORKSPACE/.ci/test_windows.ps1"
+  all-successful:
+    # https://github.community/t/is-it-possible-to-require-all-github-actions-tasks-to-pass-without-enumerating-them/117957/4?u=graingert
+    runs-on: ubuntu-latest
+    needs: [test]
+    steps:
+    - name: note that all tests succeeded
+      run: echo "🎉"