bug: systemd services do not have home.sessionVariables in their environment

[nbraud]

Are you following the right branch?

• My Nixpkgs and Home Manager versions are in sync

Is there an existing issue for this?

• I have searched the existing issues

Issue description

systemd user units do not have home.sessionVariables included in their environment, e.g. XDG_{CACHE, DATA, STATE}_HOME which are necessary for services to work properly.

The specific case of (non-default) XDG_CONFIG_HOME is out-of-scope, as it apparently cannot be properly handled in HM itself; see #5510.

Maintainer CC

@rycee (systemd)

System information

- system: `"x86_64-linux"`
 - host os: `Linux 6.6.31, NixOS, 24.05 (Uakari), 24.05pre-git`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.2`
 - channels(root): `""`
 - nixpkgs: `/run/current-system/config/3rd-party/nixpkgs`

[nbraud]

I think the most sensible approach would be to fill systemd.user.settings.Manager.DefaultEnvironment with home.sessionVariables, and I'll write I wrote a PR to that effect.