Update concurrency settings for E2E to cancel old PR-triggered runs

Define a concurrency group for the E2E workflow and configure it to
cancel in progress runs when a new run is triggered.

For PRs this means that only the latest run triggered for a given PR
will be completed as any in progress runs will be cancelled.

For push events to the `main` branch, a fallback is used (`run_id`) as
the `head_ref` is only defined for PR events. This means that we will
run the E2E to completion on each push event as expected.

.github/workflows/e2e.yml

@@ -11,6 +11,10 @@ on:
     # The branches below must be a subset of the branches above
     branches: ["main"]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 defaults:
   run:
     shell: bash