WIP: refactor CI completely

[lucacasonato]

I am going to try to split CI into distinct "build" and "test" steps. Additionally I'll attempt to shard the tests across multiple concurrent testing VMs.

TODOs:

• [ ] build in sysroot on linux
• [ ] wpt upload
• [ ] run benchmarks
• [ ] upload benchmarks
• [ ] upload release (notes and binaries)

[AaronO]

Long overdue and lots of improvements to make, looking forward to it !

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[bartlomieju]

Integration tests are split so that each file produces a separate binary - this could be done upfront in a separate PR

Done in #16816 and merged into this PR.

[bartlomieju]

Current blocker: packaging artifacts on Windows hangs.

[dsherret]

We've refactored the existing CI and it's faster so I'm going to close this one. Let's land incremental improvements to the CI on top of that work.