ArPow stage 2: implementing source-build CI

[MichaelSimons]

To make sure ArPow (arcade-powered source-build) keeps working in this repo, we need to add it to PR validation. We also need it to run in the official build to publish source-built artifacts that can be tested downstream.

Let us know when an official build runs with ArPow enabled, and the source-build team will look at it to greenlight the artifacts or make fixes.

See https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/onboarding/ci-onboarding.md for ArPow CI onboarding info.

For https://github.com/dotnet/source-build/blob/master/Documentation/planning/arcade-powered-source-build/implementation-plan.md

[MichaelSimons]

cc @JunTaoLuo

[JunTaoLuo]

Ah, I opened the same tracking issue in #31423. I'll close that one since this one has a bit more detail.

[MichaelSimons]

@JunTaoLuo, do you have an ETA on when this will be complete? This will be blocking the ArPow effort soon.

[JunTaoLuo]

It's been on my list since last week but I haven't made much progress due to other priorities. I do plan to get a PR out in the preview 5 timeline and address additional followup issues in preview 6, if any is identified.