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>

src/cmd/link/dwarf_test.go

@@ -71,8 +71,8 @@ func testDWARF(t *testing.T, buildmode string, expectDWARF bool, env ...string)
 			}
 			cmd.Args = append(cmd.Args, dir)
 			if env != nil {
-				env = append(env, "CGO_CFLAGS=") // ensure CGO_CFLAGS does not contain any flags. Issue #35459
 				cmd.Env = append(os.Environ(), env...)
+				cmd.Env = append(cmd.Env, "CGO_CFLAGS=") // ensure CGO_CFLAGS does not contain any flags. Issue #35459
 			}
 			out, err := cmd.CombinedOutput()
 			if err != nil {