cmd/go: refine GOAUTH user parsing to be more strict

cmd/go: refine GOAUTH user parsing to be more strict

This CL enhances the parsing of GOAUTH user based authentication for
improved security.

Updates: #26232

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: Ica57952924020b7bd2670610af8de8ce52dbe92f
Reviewed-on: https://go-review.googlesource.com/c/go/+/644995
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>