Shared between `ava` and `tarasco`. Also define `lib/default.nix` to share
utility functions like `usermod` between NixOS configurations.
Change-Id: I65b8d37520426e164c9d5f722bee4731a3c6e641
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5942
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>