[GH Actions] Add pull_request trigger to Actions workflows

Summary:
Add pull_request trigger to Actions workflows (except for the release workflows + harbormaster cleanup workflow).

Won't make much of a difference to developers since everything goes through Phabricator, but provides a sanity check for Dependabot PRs.

Test Plan: Will open a PR after this is landed and ensure that all the workflows complete successfully.

Reviewers: tomek, jon, varun, abosh

Reviewed By: varun

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5147