crypto/internal/cryptotest: print stderr if go tool fails
Trying to find out why "go env GOMODCACHE" is failing
on the Windows longtest builder.
For #71508
Change-Id: I0642d5a5d85a549c6edde0be5bed8f0a16cca200
Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/645895 Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>