cmd/go: don't clobber `go env GOGCCFLAGS`

cmd/go: don't clobber `go env GOGCCFLAGS`

When CC is set in the environment, the mkEnv function sets its version
of CC to the first word $CC and sets GOGCCFLAGS to the remainder. That
worked since Go 1 but was broken accidentally by
https://golang.org/cl/6409, which changed the code such that `go env`
calls mkEnv twice. The second call to mkEnv would clobber GOGCCFLAGS
based on the value of CC set by the first call. Go back to the old
handling by only calling mkEnv once.

Fixes #15457.

Change-Id: I000a1ebcc48684667e48f2b9b24605867b9e06cd
Reviewed-on: https://go-review.googlesource.com/33293
Reviewed-by: Russ Cox <rsc@golang.org>