cmd/go: code cleanups and go env output sorting

cmd/go: code cleanups and go env output sorting

Sort the table of variables in the envcmd source code,
because all tables in all source code should be sorted
unless there's a reason they can't be.

Sort the go env output as well.

Sort the flag registrations, same reason.

Remove redundant range variable.

Change-Id: I5f1af3e22a09621706eb57c369bd2675b754b063
Reviewed-on: https://go-review.googlesource.com/c/go/+/627476
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>