nixos/pam: Fix wrong string concatenation

Regression introduced by 1010271c63.

This caused the line after using the loginuid module to be concatenated
with the next line without a newline.

In turn this has caused a lot of the NixOS VM tests to either run very
slowly (because of constantly hitting PAM errors) or simply fail.

I have tested this only with one of the failing NixOS tests.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/security/pam.nix

@@ -301,11 +301,11 @@ let
            session required pam_env.so envfile=${config.system.build.pamEnvironment}
          '' + ''
            session required pam_unix.so
-         '' + optionalString cfg.setLoginUid
-              "session ${
-                if config.boot.isContainer then "optional" else "required"
-              } pam_loginuid.so"
-         + optionalString cfg.makeHomeDir ''
+         '' + optionalString cfg.setLoginUid ''
+            session ${
+              if config.boot.isContainer then "optional" else "required"
+            } pam_loginuid.so
+         '' + optionalString cfg.makeHomeDir ''
            session required ${pkgs.pam}/lib/security/pam_mkhomedir.so silent skel=/etc/skel umask=0022
          '' + optionalString cfg.updateWtmp ''
            session required ${pkgs.pam}/lib/security/pam_lastlog.so silent