Support GOTOOLCHAIN=auto

[dagood]

• The other face of Disable GOTOOLCHAIN #1213.

In theory, we could modify Go such that GOTOOLCHAIN=auto functions and installs another Microsoft Go of the needed version.

From my initial understanding of how GOTOOLCHAIN works, it includes significant checksum infra, and I'm skeptical that this is feasible for our team to implement. I'm also skeptical that it has value for the set of scenarios Microsoft Go targets, but this set may change.