`ciflow/default` should not be automatically applied for PRs that are submitted using ghstack

[seemethere]

Since ghstack creates PRs for every commit in a commit history there's potential for a large inflow of PRs to suddenly be created all requiring CI which puts a strain on our overall CI system.

We should make it so that if a PR is created using ghstack then it doesn't automatically get the ciflow/default label applied meaning the user who submitted the PR needs to explicitly label their PR to enable CI