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

cc @sfoslund

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