message: add `MSYSTEM=1` to shell_quoting test
  this should make it use unix escaping on windows
diff: @@ -28,7 +28,8 @@ fn features_are_quoted() {
  .build();
  assert_that(
  - p.cargo("check -v"),
  + p.cargo("check -v")
  + .env("MSYSTEM", "1"),
  execs()
  .with_status(101)
  .with_stderr_contains(
  