cmd/go/testdata/script: fix test failing on nocgo builders

The regression test introduced in https://golang.org/cl/318770 broke the
the nocgo builders.

Update the cgo package used in the test to ensure that it can be build
both with cgo enabled and disabled.

Change-Id: Iab0486f0b85ac5e5a22fdf8a1998edd50cbb4d96
Reviewed-on: https://go-review.googlesource.com/c/go/+/319210


Trust: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>