As part of the work for #57179 we moved configurable defaults
to GOROOT/go.env, so that packagers don't have to modify
source code to change those defaults. Since packagers may want
to modify GOTOOLCHAIN's default, move it to go.env too.
This CL modifies 'go env' to print GOTOOLCHAIN by default.
It also refines CL 496957 from yesterday to recognize any env
var in either go.env or the user go/env, not just the user go/env.
When I put GOTOOLCHAIN in go.env, but before I added it to
the default printing list, 'go env GOTOOLCHAIN' was printing
an empty string, and it was incredibly confusing.
For #57001.
Fixes #60361 while we're here.
Also includes a fix for a review comment on CL 497079 that I forgot to mail.