message: make test use nightly
diff: @@ -460,6 +460,7 @@ fn cross_doctests() {
  // This tests the library and runs the doc tests.
  p.cargo("test -v -Z doctest-xcompile --target")
  .arg(&target)
  + .masquerade_as_nightly_cargo()
  .with_stderr(&format!(
  "\
  [COMPILING] foo v0.0.1 ([CWD])
  