nixos/gdm: Conflict plymouth-quit, but more carefully

To avoid extra flickering on boot, we want GDM to tell Plymouth to
quit after GDM takes control of the display.  That configuration was
reverted in #71065 because it caused ‘nixos-rebuild switch’ to bring
down the graphical session.  The reason was that if multi-user.target
wants plymouth-quit.service which conflicts display-manager.service,
then when ‘nixos-rebuild switch’ starts multi-user.target,
display-manager.service is stopped so plymouth-quit.service can be
started.  We avoid this problem by removing WantedBy:
multi-user.target from plymouth-quit.service.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

nixos/modules/services/x11/display-managers/gdm.nix

@@ -183,14 +183,20 @@ in
       "systemd-udev-settle.service"
     ];
     systemd.services.display-manager.conflicts = [
-       "getty@tty${gdm.initialVT}.service"
-       # TODO: Add "plymouth-quit.service" so GDM can control when plymouth quits.
-       # Currently this breaks switching configurations while using plymouth.
+      "getty@tty${gdm.initialVT}.service"
+      "plymouth-quit.service"
     ];
     systemd.services.display-manager.onFailure = [
       "plymouth-quit.service"
     ];
 
+    # Prevent nixos-rebuild switch from bringing down the graphical
+    # session. (If multi-user.target wants plymouth-quit.service which
+    # conflicts display-manager.service, then when nixos-rebuild
+    # switch starts multi-user.target, display-manager.service is
+    # stopped so plymouth-quit.service can be started.)
+    systemd.services.plymouth-quit.wantedBy = lib.mkForce [];
+
     systemd.services.display-manager.serviceConfig = {
       # Restart = "always"; - already defined in xserver.nix
       KillMode = "mixed";