ArPow stage 2: implementing source-build CI

[dagood]

This depends on stage 1: #367

More background below:

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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

[dagood]

@omajid is adding this (along with stage 1) in #416.

[dagood]

Looks like it worked: https://dev.azure.com/dnceng/internal/_build/results?buildId=987670&view=results