cmd/go: fix script conditions that require cgo

cmd/go: fix script conditions that require cgo

This fixes a regression introduced in CL 419875
that causes features that require cgo to be tested
on the nocgo builders.

For #27494.

Change-Id: Iee61225c98c1275810256ab002a698fc4b42c053
Reviewed-on: https://go-review.googlesource.com/c/go/+/445235
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>