cmd/go: disable cgo by default if CC unset and DefaultCC doesn't exist
CL 621995 disrupted the behavior
introduced by CL 450739, now restore it.
Fixes #75340
Change-Id: Icd1a0eb970876995f9446e0547ceb9e78990f6ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/703555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com> Reviewed-by: Ian Alexander <jitsu@google.com> Reviewed-by: Sean Liao <sean@liao.dev>