Update `cleanup` job in `build-and-run-batch-job` workflow to run on branch deletion

[jeancochrane]

Currently we run the cleanup job in the build-and-run-batch-job workflow on closed PRs:

actions/.github/workflows/build-and-run-batch-job.yaml

Lines 320 to 323 in 17647d3

	cleanup:
	# Only run on closed PRs, to destroy staging resources
	# yamllint disable-line rule:line-length
	if: github.event_name == 'pull_request' && github.event.action == 'closed'

However, we run many of our Batch jobs on branches that we never end up putting up as PRs. As a result, we've accumulated a bunch of old Batch resources that we never use from zombie branches.

We should update our conditional logic for this job to also run on the delete event so that we can clean up resources if a branch is deleted but was never put up as a PR.