auto delete canary versions after a PR has been closed

**Is your feature request related to a problem? Please describe.**

So the problem is when you install a package maintained by auto the package packument size is really big due to all the canary builds which in turn slows down npm pulls because were tryign to read 100s of mbs. This can equate to 10s of seconds of waiting when a user has to download a package.

**Describe the solution you'd like**
We want a way to auto delete canary versions after a PR has been closed or deleted.

**Describe alternatives you've considered**
Not running `shipit` on prs

**Additional context**