Building draft PRs should be an option and not default behavior. And the default should be to NOT build drafts. This enables a PR to be updated without triggering builds after each revision. Think about it - a PR can be switched to draft to temporarily stop the upstream repo from rebuilding the PR while revisions are taking place then switched back to open when the author feels that the PR is in a proper state for the upstream repo to do their own tests.
Keep in mind - the whole point of a draft PR is to say - this PR is NOT ready for review!!!
If this becomes standard practice - everybody can save on building resources.