If one uses .deps when DEPS_BUILD_DIR is defined in another location it
leads to very surprising behaviors, as it looks for libraries in other
places other than .deps.
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.
Using the base branch as cache means that pull requests won't be able to
use the cache from the master branch, since the master branch cache
doesn't have a base_ref as it's generated from a push. Removing base_ref
makes the cache key from master and PR branch the same, provided the any
build files don't change.
