This PR aims to reduce the bottleneck of contributors waiting for CI jobs on the shared Mill repo by moving CI for draft/WIP pull requests to each contributor's own fork, each of which has a separate concurrency budget (~20 concurrent workers) from the com-lihaoyi organization.

Once a PR is marked ready-for-review, com-lihaoyi CI begins, so the author effectively has a choice of where they want to run their CI workloads. Mill's main PR validation CI jobs are portable, do not need any special secrets or environment, and can run just fine in contributor's GH Actions environments

To do this:

• Made the upstream build-linux/build-windows/test-docs CI jobs filter on github.event.pull_request.draft == false, and

• Removed the branches: - main filter on the run-tests jobs so they begin running in contributor GH Actions when commits are pushed to a branch

• Add a new Draft CI workflow that pushes a github status linking to the PR's branch in the contributor's fork, as an easy way to see the CI history for the PR there. This needs to run on pull_request_target rather than just pull_request to have the suitable permissions to update statuses

• Some subtleties around handling ready_for_review events such that when a contributor marks a PR as ready for review, CI is re-triggered and handled appropriately without needing to subsequently push a new commit.

• Substituted github.head_ref with github.ref_name in the concurrency.group key since the former only works on PR events while the latter also works on push events

This is a bit of an unusual setup AFAIK for Github Actions, but I've tested it manually a bunch and it seems to work, and should mitigate the bottleneck of multiple people updating their WIP PRs and having everyone blocked waiting on everyone else. Worst come to worst, if you mark a PR as ready_for_review it falls back to the old behavior, but this setup gives you an option to run draft PR validation on your own fork's GH Actions where it won't get blocked by other people's PRs. This is something you can actually already do today, but it requires some tedious fiddling that PR makes seamless

Tested on #3540 via git commit -am . --allow-empty && git commit -am . --allow-empty && git push origin head && git push upstream head~1:test-non-draft && git commit -am . --allow-empty && git push origin head, and targeting this PR against the test-non-draft branch in the Mill repo