cmd/go: print toolchain switching with GODEBUG=toolchaintrace
This CL introduces the ability to print information about the toolchain switch used in the
go command, controlled by the `toolchaintrace` setting. This setting defaults to `toolchaintrace=0`,
meaning no information is printed. Setting it to `toolchaintrace=1` will cause the go command
to print a message indicating the toolchain used and where it was found.
Fixes: #63939
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: Idc58e3d5bc76573aa48e1f7df352caa13004c25e
Reviewed-on: https://go-review.googlesource.com/c/go/+/610235 Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>