CI: Cancel obsolete concurrent GitHub Actions workflow jobs (#1940)

If a new push happens to a staging branch or a PR branch, continuing to
run now-obsolete tests is pointless and wasteful. However, we do want to
finish any jobs running on previous master branch commits, so that every
master branch commit has full test results.

.github/workflows/quick.yaml

@@ -16,6 +16,12 @@ on:
     # test PRs targeting this branch code
     branches: [ "master" ]
 
+concurrency:
+  # Cancel ongoing tests in case of push to the same PR or staging branch,
+  # but let previous master commit tests complete.
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
 env:
   # empty except for pull_request events
   PULL_REQUEST_NUMBER: ${{ github.event.pull_request.number }}

.github/workflows/slow.yaml

@@ -14,6 +14,11 @@ on:
   push:
     branches: [ "auto" ]
 
+concurrency:
+  # Cancel ongoing tests in case of push to staging branch.
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   linux-distros: