Automatically cancel in-progress CI runs of old commits

[seisman]

Description of proposed changes

Add the Cancel Workflow Action action as the first step for testing.

When a new commit is pushed, the previous runs in the same PR will be cancelled. So we don't have to wait for too long time for the previous outdated CI jobs.

Fixes #543

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[seisman]

Look at the two runs: old runs and new current runs. I think it works well.

[weiji14]

This is great! Thanks @seisman, you're really becoming an expert on Github Actions 😆

[seisman]

Thanks for the good and easy-to-read documentation of most GitHub Actions!