Github PR base update doesn't update branch base in appveyor

[zachdworkin]

This flow is broken

1. Open a PR where the target branch is "main"
2. Let appveyor queue up the new job
3. Switch the PR branch target to "not_main"
4. The appveyor job will remain queued and will not update the branch to "not_main"

Expected behavior
4. appveyor job will cancel and requeue with the correct branch "not_main"

The current fix is to close PR and re-open PR and let appveyor requeue the job.
If this is expected then I can open a feature request instead of a bug report