Having separate directory location causes failures to be inconsistent
and ultimately confusing. A common problem is a file with a particular
name is searched for the entire repository, which gives different
results if the dependency directory is inside the neovim directory or
outside of it.
Having to specify CI_BUILD for every CI job requires boilerplate. More
importantly, it's easy to forget to enable CI_BUILD, as seen by
8a20f9f98a. It's simpler to remember to
turn CI_BUILD off when a job errors instead of remembering that every
new job should have CI_BUILD on.
Multi-config generators can be tricky so testing them would be good.
Also test GCC release and MinSizeRel build types as they're prone to
unusual warnings. Remove release testing from test.yml as this is a
sufficient replacement.
Having a workflow that only builds neovim without running all of the
tests is a cheap way to test the build still works without burning too
much CI time.
