cmd/go: fix two-step toolchain upgrade through go install, GOTOOLCHAIN
If we do one upgrade because of a go install target's go.mod file,
we still might need a second upgrade to implement the GOTOOLCHAIN
minimum. Instead of allowing a two-step switch (which we were
cutting off anyway), skip the first step and go straight to the
GOTOOLCHAIN min upgrade.
Fixes #69051.
Change-Id: I16f060f473574d8b8f84c55fae2fd0cdabc8aa19
Reviewed-on: https://go-review.googlesource.com/c/go/+/637496 Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>