Make SHA in automerge workflow conditional on triggering event

[kvfairchild]

The automerge workflow is triggered by pull_request events, status events, and check_run events. The workflow needs access to the last commit SHA of the PR that is modifying plugins, which for status and check_run events is accessible via the default environment variable GITHUB_SHA. However, for pull_request events this variable points to the merge commit instead. We therefore use github.event.pull_request.head.sha when the workflow is triggered by a pull_request event.