cmd/link: fix data race in testDWARF

Multiple instances of testDWARF run in parallel, with a shared
backing store of the env input slice. Do modification of the
environment locally, instead of on the shared slice.

Fixes #38265.

Change-Id: I22a7194c8cd55ba22c9d6c47ac47bf7e710a7027
Reviewed-on: https://go-review.googlesource.com/c/go/+/227342


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>