Build/Test Tools: Cancel previous active workflow runs for pull requests.

Because workflow results are reported for each commit, it’s important to let all runs against main and version branches to complete so that the checks are reported accurately.

When considering and reviewing pull requests, the only workflow run that matters is the most recent.

Props ocean90, helen.
See #50401.

git-svn-id: https://develop.svn.wordpress.org/trunk@49244 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/javascript-tests.yml

@@ -11,6 +11,7 @@ jobs:
   # Runs the QUnit tests for WordPress.
   #
   # Performs the following steps:
+  # - Cancels all previous workflow runs for pull requests that have not completed.
   # - Checks out the repository.
   # - Logs debug information about the runner container.
   # - Installs NodeJS 12 (todo: install the version of NPM specified in the `.nvmrc` file to support older branches)
@@ -23,6 +24,12 @@ jobs:
     name: QUnit Tests
     runs-on: ubuntu-latest
     steps:
+      - name: Cancel previous runs of this workflow (pull requests only)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: styfle/cancel-workflow-action@0.5.0
+        with:
+          access_token: ${{ github.token }}
+
       - name: Checkout repository
         uses: actions/checkout@v2