CI: Reuse existing build for WPT runs

[nicoburns]

Background

Currently WPT tasks run on CI will all individually rebuild servo even though they run after a build task which will have already built a Servo binary for that commit. This effectively doubles the amount of time spent compiling Servo, and as this is typically slower than running the tests themselves has a significant affect on the overall amount of time it takes to run WPT checks on CI.

Proposal

• Save the binary from "build" runs as a Github Actions artefact.
• When running WPT tasks, search for artefacts matching the current commit. And if one exists use it instead of rebuilding

Notes

• This API can be used to list artefacts https://docs.github.com/en/rest/actions/artifacts?apiVersion=2022-11-28#list-artifacts-for-a-repository

[sagudev]

Artifacts are already made in build job all the time no matter if WPT is requested or not. So we only need to "skip" build task if it is already running (there is concurrency), but we need to make sure it has the same profile as we are now trying to run.

Although I am not sure when this would be useful as I usually do mach try before or immediately after pushing commit (so way before any build artifact is actually available).