cmd/go: split quotes in GOFLAGS same as in other env vars

cmd/go: split quotes in GOFLAGS same as in other env vars

GOFLAGS didn't split on quotes because no other env vars
(such as CC, CXX, ...) did either. This kept them all consistent.

CL 341936 changed everything but GOFLAGS, making them inconsistent.

Split GOFLAGS the same way as the other environment variables.

Fixes #26849.

Change-Id: I99bb450fe30cab949da48af133b6a36ff320532f
Reviewed-on: https://go-review.googlesource.com/c/go/+/443956
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>