fix(nixos): expose configured sessions to greeter

[ooonea]

Summary

Expose the configured NixOS Wayland sessions to Noctalia Greeter.

Motivation

NixOS keeps display-manager session entries in a generated aggregate that is not linked into the normal system profile. As a result, the greeter cannot discover configured sessions through /run/current-system/sw/share/wayland-sessions.

Prefix the greetd command with XDG_DATA_DIRS pointing directly at services.displayManager.sessionData.desktops, using the packaged env executable.

Type of Change

• Bug fix
• Build / packaging

Testing

• nix develop -c just format
• Evaluated a NixOS configuration with a synthetic Wayland session and verified that the generated greetd command contains the exact session aggregate path.

Manual Coverage

• Tested under greetd (real login flow)
• Tested with just run / just run-local (dev compositor)
• Tested with multiple monitors
• Tested appearance sync from Noctalia Shell (Sync Now)
• Tested on NixOS (programs.noctalia-greeter)
• Tested with a pinned [output].name
• Tested with custom [output].layout / [output].transforms

Checklist

• This PR is ready for review, or it is marked as Draft.
• I read and followed the relevant guidance in CONTRIBUTING.md and README.md.
• I ran just format with clang-format v22+ installed, or this PR has no code changes.
• I ran the relevant build or test commands, or explained why they were not run.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I updated user-facing documentation in docs/user/ when this PR changes user-visible behavior or configuration, or this PR does not require documentation changes.
• I used the existing canonical names for config keys, paths, and identifiers.