message: fix short_message_format vs nightly
diff: @@ -724,6 +724,10 @@ fn check_artifacts() {
  #[test]
  fn short_message_format() {
  + if !is_nightly() {
  + // This can be removed once 1.30 is stable (rustdoc --error-format stabilized).
  + return;
  + }
  let foo = project()
  .file("src/lib.rs", "fn foo() { let _x: bool = 'a'; }")
  .build();
  