nixos/postgresql: Fix execution of initialScript.

Regression introduced by b21fd5d066.

The initialScript is only executed whenever there is a .first-startup in
the dataDir, so silently dropping the file essentially breaks
initialScript functionality.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
(cherry picked from commit 60d407b209)

nixos/modules/services/databases/postgresql.nix

@@ -202,6 +202,8 @@ in
                   # For non-root operation.
                   initdb
                 fi
+                # See postStart!
+                touch "${cfg.dataDir}/.first_startup"
             fi
 
             ln -sfn "${configFile}" "${cfg.dataDir}/postgresql.conf"