cmd/go: do not append to the global cfg.OrigEnv slice

Appending to a global slice is only safe if its length is already
equal to its capacity. That property is not guaranteed for slices in
general, and empirically does not hold for this one.

This is a minimal fix to make it easier to backport.
A more robust cleanup of the base.EnvForDir function will be sent in a
subsequent CL.

Fixes #38077
Updates #37940

Change-Id: I731d5bbd0e516642c2cf43e713eeea15402604e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/225577


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>