systemd: Add user environment generator for session variables

[BohdanTkachenko]

Description

On systems where graphical sessions like GNOME are started by the systemd user manager, the session environment is not sourced from a traditional shell profile. This results in an incomplete PATH environment variable. Consequently, .desktop files for applications installed by Home Manager are considered invalid because their Exec= target cannot be found in the PATH, and the applications do not appear in menus.

While systemd.user.sessionVariables can set simple variables, it is not well-suited for dynamically constructing PATH. Systemd provides a more suitable mechanism for this task: an environment generator.

This commit implements this solution by adding a script to ~/.config/systemd/user-environment-generators/. This script is executed by the systemd user manager on startup and sources:

• The main Nix profile script (nix.sh)
• The Home Manager session variables script (hm-session-vars.sh)

This correctly populates the environment for all services and applications managed by the user instance. While the primary motivation is fixing the PATH, this approach also ensures all other environment variables from the profile scripts are set. This makes the systemd session environment consistent with that of an interactive shell profile, providing a more predictable experience regardless of how the user session is initiated.

A test is included to validate the generator's creation and content.

Fixes: #1439

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[BohdanTkachenko]

It looks like $XDG_CONFIG_HOME/systemd/user-environment-generators is actually not a standard path for these generator scripts and it is not listed in systemd.environment-generator(7).

There is an open feature request systemd/systemd#32423 to add this path by default and there is also a workaround, which I am going to try to use.

I discovered this, because this pull request works on my laptop, but does not work on my PC. I have no idea why it works on my laptop. On my laptop I use bluefin-dx and on my PC I use bazzite, which is based on Bluefin as well. I checked Bazzite and Bluefin repos but couldn't find anything that would affect this behavior. What is even more confusing is that systemd-path systemd-search-user-environment-generator does not list $XDG_CONFIG_HOME/systemd/user-environment-generators on both my PC and my laptop, so it shouldn't work! I tried grepping configs on my laptop to find where user-environment-generators is referenced and was only able to find it in the same basic libs as on my PC. Kind of a mystery here...

This PR probably cannot be merged until we can figure out how to make it reliably work on most environments, or until systemd/systemd#32423 is addressed.

[BohdanTkachenko]

So the workaround suggested in systemd/systemd#32423 (comment) worked, so I added a systemd config file to override default generators path.

Regarding to why previously I though it is working was trivial - during previous debug session I hardcoded something to make it work and then forgot about that.

[teto]

sounds promising. not sure when I can test but will do