ci(pr): checkout base_ref - #1357

This may not be what is wanted. What if the PR is behind master?

A valid solution might be to check out master every time instead?

I think checkout out the default branch head commit makes sense, and I thought that is what it was doing, unless there is a delay with the pull_request_trigger trigger

It looks like github.base_ref might work. https://docs.github.com/en/free-pro-team@latest/actions/reference/context-and-expression-syntax-for-github-actions

Doesn't look like it likes it https://github.com/nodejs/docker-node/runs/1223378399?check_suite_focus=true