Don't run CI for draft PRs

[mark-i-m]

I would like to propose that CI should not be run on PRs that are marked as "Drafts" using the Github "Drafts" feature.

Run CI wastes resources and produces lots of noisy messages from rust-highfive bot about failing CI.

[joshtriplett]

It should be possible to explicitly ask for them to run through CI, but it makes sense to not run CI on drafts by default.

[mark-i-m]

It might be a bit heavy-weight, but I think we can already use bors try for that.

[mark-i-m]

It looks like Travis, Azure, and Github don't have support for such a feature yet...

[steveklabnik]

Triage: this looks to be possible for Github Actions? https://github.community/t5/GitHub-Actions/Don-t-run-actions-on-draft-pull-requests/m-p/55316#M9508

[pietroalbini]

We discussed this in the infrastructure team meeting and decided not to go forward with this approach. You can see the context on the decision by reading the contents of the meeting.