nixos/lightdm: make lightdm user shell bash

In https://github.com/NixOS/nixpkgs/issues/100119 pantheon's greeter
has g-s-d running which allows brightness controls via pkexec.
This is changed in newer versions of g-s-d (pantheon uses a fork currently),
but whenever brightness is changed with a shell of `shadow` we get
```
Oct 10 23:51:44 kirXps pkexec[18722]: lightdm: Executing command [USER=root] [TTY=unknown] [CWD=/var/lib/lightdm] [COMMAND=/run/current-system/sw/bin/elementary-settings-daemon/gsd-backlight-helper /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight 65587]
```

I'm not sure this should be strictly needed, so we should try to
revert later on when pantheon's g-s-d is updated.

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

@@ -308,6 +308,7 @@ in
       home = "/var/lib/lightdm";
       group = "lightdm";
       uid = config.ids.uids.lightdm;
+      shell = pkgs.bash;
     };
 
     systemd.tmpfiles.rules = [