nixos/gdm: Fix pulseaudio tmpfiles structure

Fix the following startup failure of the sound service in the gdm
session that was introduced by NixOS#75893:
```
Feb 16 11:44:15 qp pulseaudio[1432]: W: [pulseaudio] core-util.c: Failed to open configuration file '/run/gdm/.config/pulse//daemon.conf': Not a directory
Feb 16 11:44:15 qp pulseaudio[1432]: W: [pulseaudio] daemon-conf.c: Failed to open configuration file: Not a directory
Feb 16 11:44:15 qp systemd[1380]: pulseaudio.service: Main process exited, code=exited, status=1/FAILURE
Feb 16 11:44:15 qp systemd[1380]: pulseaudio.service: Failed with result 'exit-code'.
Feb 16 11:44:15 qp systemd[1380]: Failed to start Sound Service.
```

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

@@ -168,7 +168,8 @@ in
     systemd.tmpfiles.rules = [
       "d /run/gdm/.config 0711 gdm gdm -"
     ] ++ optionals config.hardware.pulseaudio.enable [
-      "L+ /run/gdm/.config/pulse - - - - ${pulseConfig}"
+      "d /run/gdm/.config/pulse 0711 gdm gdm"
+      "L+ /run/gdm/.config/pulse/${pulseConfig.name} - - - - ${pulseConfig}"
     ] ++ optionals config.services.gnome3.gnome-initial-setup.enable [
       # Create stamp file for gnome-initial-setup to prevent it starting in GDM.
       "f /run/gdm/.config/gnome-initial-setup-done 0711 gdm gdm - yes"