Source-build template default pool fix; clean up parameters

[dagood]

Fixes dotnet/source-build#1795 by always setting the vm image to ubuntu-20.04 by default.

Incorporates suggestions from @tmat based on dotnet/sourcelink#690 (thanks!):

• Rename runSourceBuild -> enableSourceBuild.
  • Also bring the parameter up closer to the enable properties away from the other run property. enable is much more common for this kind of flag and will fit in nicer.
• Use default platform if and only if 0 platforms are passed.
  • This simplifies the implementation from 3 lines to 1 line of code for ordinary, managed-only, arcade-using repos.
  • This removes the ability to use the default platform if you're also passing in your own platforms. However, this functionality is probably not useful enough to justify the extra lines in ordinary repos. If a repo defines its own platforms, it's only a few more lines to include a copy of the default platform. (This copy won't receive auto-updates, but the custom platforms won't receive auto-updates either, so this seems like a non-issue in context.)
    • It might not be hard to bring this back with some extra conditions, but it doesn't seem worth the yml template bloat.

Only one repo uses these templates/parameters so far (dotnet/source-build-reference-packages) so I'm not concerned about this being a breaking change.