devshell: `act` to run GitHub workflows locally

[michalrus]

Context

• As our workflows are getting more complicated, it would be good to be able to run them locally.

• There exists https://nektosact.com/ · https://github.com/nektos/act

• Does it work on macOS? Supposedly – using Docker

• How does it work on Linux? Probably Docker too

• There exists a Nix package called act – test it locally first:

  ❯ nix eval --impure --expr '(import <nixpkgs> {}).act.meta.homepage'
  "https://github.com/nektos/act"