cmd/dist: simplify cgo_test
cmd/dist: simplify cgo_test

Now that misc/cgo/test doesn't invoke any Go builds itself, we don't
need to use GOFLAGS to thread build flags down into sub-builds.

Preparation for #37486.

Change-Id: I9ecdd215da5c21339cbf76765f8be2190d19539c
Reviewed-on: https://go-review.googlesource.com/c/go/+/447356
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go