[ArPow] Onboard test-templates to arcade-powered source-build

[crummel]

No description provided.

[omajid]

PR: dotnet/test-templates#126

[omajid]

PR was merged last week.

[dagood]

Here's an internal/official build that ran with that change that we can run through the source-build-side stage 3: https://dev.azure.com/dnceng/internal/_build/results?buildId=995058&view=results

[dagood]

I took a look, and the artifacts look reasonable. The prebuilt report is pointing out that it seems to be missing <SourceBuild...> elements to grab ArPow-built arcade, ref packages, sourcelink, and xliff-tasks. Everything there except the Arcade one requires darc, to set up subscriptions, so I think it's reasonable to leave it as it is for now and burn down these prebuilts when we get to stage 4.

(In case I haven't mentioned it elsewhere before, these aren't going to necessarily end up as prebuilts in the final tarball: the inputs and flow being more strict should force source-build usage just like it does in dotnet/source-build.)