Move check_commits CI to dedicated workflow (main)

[tnull]

Previously, we introduced a chance dynamically determining the base
branch for the check_commits CI job. Unfortunately it used the base_ref
variable, which is only set for pull_requests, not for pushes. Here, we
hence move check_commits to a dedicated workflow that only is run on
PRs.

[ldk-reviews-bot]

🎉 This PR is now ready for review!
Please choose at least one reviewer by assigning them on the right bar.
If no reviewers are assigned within 10 minutes, I'll automatically assign one.
Once the first reviewer has submitted a review, a second will be assigned if required.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.92%. Comparing base (1e0b75e) to head (9893797).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3966      +/-   ##
==========================================
- Coverage   88.93%   88.92%   -0.01%     
==========================================
  Files         174      174              
  Lines      123869   123876       +7     
  Branches   123869   123876       +7     
==========================================
- Hits       110161   110158       -3     
- Misses      11256    11258       +2     
- Partials     2452     2460       +8     

Flag	Coverage Δ
fuzzing	22.17% <ø> (-0.01%)	⬇️
tests	88.75% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ldk-reviews-bot]

👋 The first review has been submitted!

Do you think this PR is ready for a second reviewer? If so, click here to assign a second reviewer.