Create an arm64 CI leg

[benvillalobos]

I'm assuming we have plenty of unit tests for this on framework/core. Related question: should we have an ARM leg for CI? Seems like it would be helpful here.

_Originally posted by @Forgind in https://github.com/dotnet/msbuild/pull/7736#pullrequestreview-1020327654_

This is probably for the best in the long run.

[Forgind]

I brought this up recently. Sounds like it's too expensive in the immediate future.