Github actions: faster build

Stream: general

[Apurva Nakade]

I just want to say thanks to whoever made changes to the Github build check actions and made it incremental. Now my code gets checked in 8 minutes instead of 8 hours. This is incredible! Thanks so much!!

[Floris van Doorn]

It was already incremental, but now we're running them on much faster machines.

[Floris van Doorn]

Thanks to @Johan Commelin