Split build/test workflows into internal and external

Currently we do some hacks with pull_request_target to allow our build/test workflows to work for external PRs. This has some irritating drawbacks for internal PRs, however:

Changes to workflows only reflect after they are merged into main.
A ok-to-test label has to be manually added to get tests to run for a PR.

We should be able to fix this with a separate internal pull_request workflow and external pull_request_target workflow.

See: https://sameproject.slack.com/archives/C01LNQJ3XHU/p1652098695858589 https://github.community/t/github-actions-are-severely-limited-on-prs/18179/17

SAME-Project / same-project

Split build/test workflows into internal and external #131