Motivation for this change

Things done

• [x ] Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • [x ] NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• [ x] Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• [ x] Ensured that relevant documentation is up to date
• [ x] Fits CONTRIBUTING.md.

Notify maintainers

cc @c0bw3b

I have added the MILL_VERSION variable, documented here:

• https://www.lihaoyi.com/mill/index.html

It seems there is an idiom around this script:

• https://github.com/lihaoyi/fastparse/blob/master/mill

I hope this avoids some of the downloading that occurs, and was mentioned in previous update.

I have also added java command to the paths, as it seems required for running some of the tests (I've only tested them on Lihaoyis own repositories that uses mill)

Besides this, I hope this should work without any issues.