Add linting for GitHub actions

[czunker]

Is your feature request related to a problem? Please describe.
With #556, I removed the act check for our workflows, because it does not support re-usable actions.

But linting our workflow is still helpful. Perhaps this linter helps: https://github.com/rhysd/actionlint

Describe the solution you'd like
Linting of our workflows.

Describe alternatives you've considered
I only know of act and the linked linter.