nixos/nvidia: intermittent suspend resume failure on proprietary drivers due to systemd freezing

[epetousis]

Describe the bug

Occasionally when suspending my RTX 3080 based system while using the Nvidia proprietary drivers, systemd-sleep will fail to freeze the user slice before sleeping:

Jan 05 12:42:28 evan-pc systemd-sleep[63989]: Failed to freeze unit 'user.slice': Connection timed out
Jan 05 12:42:28 evan-pc systemd-sleep[63989]: Performing sleep operation 'suspend'...
Jan 05 12:42:28 evan-pc kernel: PM: suspend entry (deep)

This appears to cause Gnome to lock up when resuming from suspend. I often end up force rebooting out of laziness, rather than attempting to recover the system.

@MinerSebas raised this a few months ago as it was mentioned as a known issue in the systemd 256 changelogs, but it appears to have fallen by the wayside (possibly because everyone is opting for the open drivers?).

It seems like the solution is to apply the changelog's suggested environment variable to the systemd-suspend service:

systemd.services.systemd-suspend.environment.SYSTEMD_SLEEP_FREEZE_USER_SESSIONS = "false";

So far I am not running into the issue after adding this to my system module. I would assume this should go into the Nvidia module and be enabled when hardware.nvidia.open = false, but the maintainers may have other thoughts on this.

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 6.12.2-zen1, NixOS, 25.05 (Warbler), 25.05.20241211.5d67ea6`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Lix, like Nix) 2.91.1
System type: x86_64-linux
Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux
Features: gc, signed-caches
System configuration file: /etc/nix/nix.conf
User configuration files: /home/epetousis/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/epetousis/.local/share/flatpak/exports/etc/xdg/nix/nix.conf:/var/lib/flatpak/exports/etc/xdg/nix/nix.conf:/home/epetousis/.nix-profile/etc/xdg/nix/nix.conf:/nix/profile/etc/xdg/nix/nix.conf:/home/epetousis/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/epetousis/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf
Store directory: /nix/store
State directory: /nix/var/nix
Data directory: /nix/store/984rvyvf2mvwhgyx7bwhrjvsgv6ln42d-lix-2.91.1/share`
 - nixpkgs: `/nix/store/rhn4fkh7hqjlqs9j6naaq623qpl62yz0-source`

Notify maintainers

@Kiskae @edwtjo

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