nixos/display-managers: don't force graphical-session.target activation for UWSM sessions

[Scrumplex]

UWSM already handles graphical-session.target.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

Add a 👍 reaction to pull requests you find important.

[Scrumplex]

CC @Vladimir-csp

Do you think there is a better way to detect UWSM than checking for one of its env vars?

[Vladimir-csp]

When is this code being executed? If during session startup, then maybe something like this may be tested on being non-empty: systemctl --user list-units --state=active,activating -ql --plain wayland-wm-env@*.service wayland-wm@*.service

[Scrumplex]

The script this is used in is called as a session wrappper by sddm, ly and some other DMs.

I think we might not be able to easily detect uwsm here, as it is run before exec-ing the actual WM/DE