fix(semver-checks): Post comments on pull_request_targets

[aborgna-q]

#22 changed the recommended trigger for the semver checks action to pull_request_target, but inside the action we were still checking for pull_request targets explicitly so the comments never got posted.

Note that for both events the payload is still called pull_request, so there's no need to change the rest of the code.
https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#pull_request_target