Make debug tmate step a dropdown instead of a checkbox

[bkiahstroud]

Currently, checking the debug checkbox when manually starting the build-test-lint action will stop every step and open a tmate session. This is inconvenient if a dev is trying to debug the test step, for example, because they would have to first tmate into the build step and close the session to allow the build to proceed.

A dropdown that allowed the dev to select which step they want to debug (build, test, or lint) would be a much smoother experience.

Related convo in Slack