ci: fix cross testing

[BurntSushi]

I have no idea how CI was passing with this error in the CI
configuration. Apparently it is failing the master run, but the PRs
were not failing. Crazy.

[amarshall]

GitHub Actions jobs with job definition errors simply don’t run, rather than be failed. Since the check was not required, it shows “green” on the PR (which only shows individual jobs) but red overall on the run page that also shows the syntax errors. The typical way to fix this is to make the job or a dependent of it a required check.

[BurntSushi]

Thanks for adding context!

Do you mean this? I don't have access to do that on this repo. Everything is very stripped down in the rust-lang org. I'd probably have to go pester someone to do that.

Regardless, look at this page: https://github.com/rust-lang/regex/pull/1282/checks

If you look very closely, you can see there are "2 errors" under "Annotations". And there is a little ! icon with the number 5 next to the ci on:pull_request area. But those, as far as I can see, are the only indicators that anything has gone wrong. There are the missing jobs, but it's hard for the eye to notice what is missing. Crucially, there is a big green check mark in the top left corner. That to me says, "everything is okay" and then my eyes move on. This is absolutely terrible UI design.