Investigate and fix MSBuild smoke test issues on Linux

[pjcollins]

Context: #5681 (comment)

We should extend the test coverage of our Linux build and smoke test job.

[jonpryor]

Fixed by? 1b71da0

[pjcollins]

Commit 1b71da0 moved Linux builds to a new 1ES Managed machine pool, but didn't change build/test scope at all. Extending test coverage for Linux should still be looked at eventually if we want to promote the build artifacts being created there.

[pjcollins]

The smoke test issues mentioned in #5681 are fixed by #6585, and I will plan to separate out Linux tests into a separate job.