.github: cancel CI run for stale PR commits (#34964)

Each commit on a PR kicks off a CI run. Those CI jobs run to the finish
regardless, even when new commits have been pushed which make them stale
and useless. This change attempts to cancel any previously running job
for the same PR.

.github/workflows/go.yml

@@ -7,6 +7,11 @@ on:
       - master
   workflow_dispatch:
 
+# Free runner capacity by cancelling superseded PR runs.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   lint:
     name: Lint