This is an optimization for the release CI. The release CI can take
a while since it compiles release builds for all operating systems.
We cut down on duplicate work and overall time by fetching
tree-sitter grammar repositories and then using those repositories
in all later steps. Previously we built all of helix just to run
helix_loader::grammar::fetch_grammars()
which is wasteful on time. With this change we only build the
helix-loader crate.
