nixos/gdm: Fix pulseaudio tmpfiles structure (#80274)

* nixos/gdm: Fix pulseaudio tmpfiles structure

Fix the following startup failure of the sound service in the gdm
session that was introduced by #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.
```

Co-authored-by: worldofpeace <worldofpeace@protonmail.ch>