Use tmp/nginx since nginx appears to create this dir

nginx/conf/nginx.conf.example

@@ -21,7 +21,7 @@ http {
   keepalive_timeout  65;
   #gzip  on;
-  proxy_temp_path /home/git/gitlab/tmp;
+  proxy_temp_path /home/git/gitlab/tmp/nginx;
   upstream gitlab {
     server unix:/home/git/gitlab/tmp/sockets/gitlab.socket fail_timeout=0;