cmd/go: move gc concurrency level computation near gcflags

So after constructing "args" variable, "gcflags" is not used anywhere.
It makes the code easier to maintain, and prevent subtle bug like #48490.

Change-Id: I41653536480880a8a6f9fbf6cfa8a461b6fb3208
Reviewed-on: https://go-review.googlesource.com/c/go/+/351849


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>