Illumos CI

[tertsdiepraam]

As came up in #5489, we'd love to have CI for Illumos!

Based on the advice from @Toasterson (#5489 (comment)), that should probably be done using a Vagrant box:

https://github.com/twpayne/chezmoi/blob/v2.9.3/.github/workflows/main.yml#L173-L191

Running the CI like that probably will uncover some issues, which means that we'll have to disable some utils and tests (temporarily) or fix the issues.

[bbarker]

I can confirm there are slightly more than 70 tests failing on illumos (all outside of uucore though) - I haven't even begun to look into these yet. I do think getting the CI working should take priority over fixing these individually, so I'll look into that.