modules/common/user: refer to users.‹username›.name as the username

[GetPsyched]

Hjem assumes that username in hjem.users.‹username› is the actual username of the user, which is true in most cases, except when users.users.‹name›.name is set to a different value.

This causes hjem-activate@ to fail because it tries to determine the user based on attrNames hjem.users.

Oct 26 02:40:38 piglin systemd[1]: Starting Link files for primary from their manifest...
Oct 26 02:40:38 piglin (e_-start)[31313]: hjem-activate@primary.service: Failed to determine user credentials: No such process
Oct 26 02:40:38 piglin (e_-start)[31313]: hjem-activate@primary.service: Failed at step USER spawning /nix/store/9x329kaihmbj82m74q7mb1ra2vadba39-unit-script-hjem-activate_-start/bin/hjem-activate_-start: No such process

This patch adds hjem.users.‹username›.name as a configurable option, and uses users.users.name as its default (nixos and nix-darwin alike).

Note that I don't know if I've covered all of the places where a change should be made, which won't be obvious from the PR diff. Happy to go back and add changes in such places if so.

Sanity Checking

• My changes fit the guidelines found in CONTRIBUTING guide
• I have tested, and self-reviewed my code
• The unit tests for Hjem pass (nix flake check/nix-build -A checks)
• Style and consistency
  • I formatted all relevant code (nix fmt/nix run -f . formatter)
  • My changes are consistent with the rest of the codebase
  • My commit messages fit the guidelines found in CONTRIBUTING
• If new changes are particularly complex:
  • My code includes comments in particularly complex areas
  • I have included a section in the documentation
• Tested on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin

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[GetPsyched]

I've set a default here, but also in L98 (modules/nixos/base.nix). Either one of these is probably redundant. What should I do?

[NotAShelf]

I'm under the impression that L98 in modules/nixos/base.nix is the redundant one, and it's also implicit. Technically, anyway.

However I do not see an issue with keeping both for the sake of consistency.