users-groups module: use buildEnv in per-user profiles

[michalrus]

Motivation for this change

• Resolves users-groups module: per-user packages should use buildEnv instead of symlinkTree. #31253 (uses buildEnv with correct inherits),

• modified naming, more in line with what nix-env -i does: cf. /nix/var/nix/profiles/per-user/$USER/profile and /nix/store/vibi47czqhjbsakwny7zpd6nqsbvqkx7-user-environment,

• $LOGNAME was used only once (in this place) in the whole Nixpkgs, we tend to use $USER instead.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[michalrus]

/cc @4z3 @Mic92 @edolstra @garbas @fpletz @nbp @edef1c

[4z3]

👍 for changing to buildEnv if that's the right way to do it (but it would be nice to keep the user's name in the derivation's name). I'm indifferent about the change of the path within /etc. I don't like changing LOGNAME to USER, because the former is defined by POSIX (though, I know we don't care in nixpkgs).

[michalrus]

Thank you very much!

[vcunat]

Is this too intrusive to pick into 17.09? #32728

[michalrus]

It would be wonderful. ♥

[roberth]

What's the reasoning for applying system.path.postBuild to user environments instead of just the system path?