Test build is dozens of gigabytes and millions of files and takes 20+ minutes.

When setting up github actions matrix, we have a choice of either (1) build before every test (+20 minutes per test) or (2) making a huge docker image and upload it, and download it for each worker, which also is 20+ minutes or more.

You can use github actions if you just use github to allocate nodes, and do all the work with your custom test scheduler, but then you are starting on the road of rewriting the CI/CD for yourself.

There are limits on build queue sizes unless you buy the enterprise plan. Also doesn't work if you are not hosting code on GitHub