Add CS workflow

[sanmai]

This PR:

• Adds code style checks since Travis CI is out of use.
• Uses all necessary caching to ensure fastest builds.
• This is a follow-up to Add CS workflow #83, for which GitHub can't handle update

GitHub actions might need to be enabled for this repository, but this is as hard as checking a checkbox.

There's a slight annoyance with GitHub actions: they won't run a workflow before you have just one enabled.