Configure checkout for use with pull_request_target

[msfjarvis]

I have had to switch my workflows to use pull_request_target so they could access specific secrets, but it resulted in checkout cloning my main branch rather than the PR head which means obviously broken pull requests also don't fail any tests because the code doesn't get tested. Is there a way to fix this problem short of returning to pull_request?

[msfjarvis]

Fixed by passing the merge ref to checkout android-password-store/Android-Password-Store#1247