chore(testing): validate requirements lockfiles with a github action

[emmettbutler]

This change adds a github action that fails when a run of scripts/compile-and-prune-test-requirements leaves a dirty git tree, indicating that requirements lockfiles need to be generated or updated to match riotfile changes on the branch in question.

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed.
• Documentation is included (in-code, generated user docs, public corp docs).
• PR description includes explicit acknowledgement/acceptance of the performance implications of this PR as reported in the benchmarks PR comment.

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.

[emmettbutler]

This is failing due to actions/checkout#363