Support .dir-locals.el

It would be nice if settings from `.dir-locals.el` were applied. elisp-lint supports this, and I need it for a couple checkers to prevent failure.

I'm trying it out and it seems to work well. nix-emacs-ci is really nice. I didn't know about github actions either. I'll have to try that too at some point. Would be nice to also be able to do CI testing on OSX. I'll see if I can get that working without having to change anything in the script.