cmd/go: skip TestDocsUpToDate if 'go build' is not supported

cmd/go: skip TestDocsUpToDate if 'go build' is not supported

testGo is currently only configured if testenv.HasGoBuild returns
true, which implies that a complete toolchain is present.
Since setting up testGo now only uses the test binary itself, it does
not actually require 'go build', but fixing that will be a bit more
involved. For now, just skip the test when it isn't set up.

Fixes #58775.

Change-Id: I6487b47b44c87aa139ae11cfa44ce6f0f5f84bd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/472095
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>