nixos/mailman: store locks in ephemeral runtime directory

nixosTests.mailman: test mailman master lock handling

nixos/modules/services/mail/mailman.nix

···
         queue_dir = "$var_dir/queue";
         template_dir = "$var_dir/templates";
         log_dir = "/var/log/mailman";
-        lock_dir = "$var_dir/lock";
+        lock_dir = "/run/mailman/lock";
         etc_dir = "/etc";
         pid_file = "/run/mailman/master.pid";
       };

nixos/tests/mailman.nix

···
         wait_for_api()
         machine.succeed("curl --fail-with-body -sLSu restadmin:secretpassword http://localhost:8001/3.1/domains")
         machine.succeed("curl --fail-with-body -sILS http://localhost/")
+
+    with subtest("service locking"):
+        machine.fail("su -s /bin/sh -c 'mailman start' mailman")
+        machine.execute("systemctl kill --signal=SIGKILL mailman")
+        machine.succeed("systemctl restart mailman")
+        wait_for_api()
   '';
 }