attach checks to PR head ref when running in context of pull request #22
Description
Currently the action always attaches the check runs it creates to the ref that triggered the workflow (github.sha) in which the action is used. When the workflow is triggered in the context of a PR (pull_request event) the triggering ref is the merge commit and the created check runs are not visible on the PR's Conversation or Checks tabs.
The request is to change the action such that it prefers the ref from the pull_request event (github.event.pull_request.head.sha) if it's available and use the triggering ref otherwise. With this approach the checks will be properly attached to the PR that triggered the workflow in which the action was used.