cmd/compile/internal/logopt: preserve env while running command

The test was not preserving temporary directory flags leading to a
failure on windows with:

    mkdir C:\WINDOWS\go-build315158903: Access is denied.

Fixes #38251

Change-Id: I6ee31b31e84b7f6e75ea6ee0f3b8c094835bf5d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/227497
Reviewed-by: David Chase <[email protected]>

Author: egonelbre

src/cmd/compile/internal/logopt/logopt_test.go

@@ -199,7 +199,7 @@ func testCopy(t *testing.T, dir, goarch, goos, src, outfile string) (string, err
 	t.Log(run)
 	cmd := exec.Command(run[0], run[1:]...)
 	cmd.Dir = dir
-	cmd.Env = []string{"GOARCH=" + goarch, "GOOS=" + goos}
+	cmd.Env = append(os.Environ(), "GOARCH="+goarch, "GOOS="+goos)
 	out, err := cmd.CombinedOutput()
 	t.Logf("%s", out)
 	return string(out), err