create_trees: pass __PORTAGE_TEST_HARDLINK_LOCKS

This allows testing of stage builds with __PORTAGE_TEST_HARDLINK_LOCKS
set in the environment, since __PORTAGE_TEST_HARDLINK_LOCKS needs to
propagate to childs processes for all ROOTs.
diff --git a/pym/portage/__init__.py b/pym/portage/__init__.py
index 339c64f..3495b96 100644
--- a/pym/portage/__init__.py
+++ b/pym/portage/__init__.py
@@ -522,7 +522,8 @@
 		clean_env = {}
 		for k in ('PATH', 'PORTAGE_GRPNAME', 'PORTAGE_USERNAME',
 			'SSH_AGENT_PID', 'SSH_AUTH_SOCK', 'TERM',
-			'ftp_proxy', 'http_proxy', 'no_proxy'):
+			'ftp_proxy', 'http_proxy', 'no_proxy',
+			'__PORTAGE_TEST_HARDLINK_LOCKS'):
 			v = settings.get(k)
 			if v is not None:
 				clean_env[k] = v