Use a minimal binary to fetch grammar sources in release CI (#2557)

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.

helix-loader/Cargo.toml

@@ -9,6 +9,10 @@ categories = ["editor"]
 repository = "https://github.com/helix-editor/helix"
 homepage = "https://helix-editor.com"
 
+[[bin]]
+name = "hx-loader"
+path = "src/main.rs"
+
 [dependencies]
 anyhow = "1"
 serde = { version = "1.0", features = ["derive"] }
@@ -27,4 +31,3 @@ threadpool = { version = "1.0" }
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 libloading = "0.7"
-