gdm: set run-dir to prevent erroneous creation of /var/run as directory if it does not exist

closes #432251

The failure occured because:
1. Root directory has wrong permissions
2. systemd-tmpfiles does not produce necessary symlinks because of that (#432261 gets created first)
3. **gdm creates a directory as a fallback**
4. various services (such as nsncd) fail on startup

Fixing the root directory permissions later does not fix this,
because `systemd-tmpfiles` will not replace an existing directory.

Removing the erroneous `/var/run` directory on a running system does
not fix this, as gdm will recreate the directory on shutdown.

pkgs/by-name/gd/gdm/package.nix

···
     "-Dsystemduserunitdir=${placeholder "out"}/lib/systemd/user"
     "--sysconfdir=/etc"
     "--localstatedir=/var"
+    (lib.mesonOption "run-dir" "/run/gdm")
   ];
 
   nativeBuildInputs = [