Add 'build RC1 packages' to monthly release steps

doc/release/monthly.md

@@ -38,6 +38,7 @@ Xth: (6 working days before the 22nd)
 - [ ] Determine QA person and notify this person
 - [ ] Check the tasks in [how to rc1 guide](https://dev.gitlab.org/gitlab/gitlabhq/blob/master/doc/release/howto_rc1.md) and delegate tasks if necessary
 - [ ] Create CE, EE, CI RC1 versions (#LINK)
+- [ ] Build RC1 packages (EE first) (#LINK)
 
 Xth: (5 working days before the 22nd)