Github actions

Stream: lean4

[Yury G. Kudryashov]

What do you think about adding github actions for common tasks like "install elan", "fetch cache", "lake build" so that people can use them by adding something like

- name: 'Install elan'
  uses: leanprover/setup-elan-action@v1.0

instead of copy+pasting from Std/Mathlib/...?

[Johan Commelin]

I like that!

[Kim Morrison]

This is in progress, thanks to @Austin Letson, see https://github.com/leanprover/lean4/issues/3950.