cmd/fix: allow cgo commands in tests to fail if 'go build' is not supported

cmd/fix: allow cgo commands in tests to fail if 'go build' is not supported

testenv.HasCgo reports whether the test binary may have been built
with cgo enabled, but having been built with cgo does not necessarily
imply that the test can invoke the cgo tool itself.

This should fix a test failure on the android builders introduced in
CL 450714.

Change-Id: I2eed4098736e1cb285ca20bc248b0ab3515f0dea
Reviewed-on: https://go-review.googlesource.com/c/go/+/451221
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>