Use actionlint on GitHub Actions workflows

[frangio]

Actionlint is a linter for GitHub Actions workflows. It can detect security problems like:

• https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
• https://github.com/nikitastupin/pwnhub

I set this to run only when the workflows change because it won't happen often and we don't want to see the check all the time.

This is what it would look like on a workflow with errors:

Our existing workflows don't have any errors. (Great!)

One thing to note is that actions like our setup action aren't linted. There is a feature request for this (rhysd/actionlint#46).