Implement a more robust way of locating rust-analyzer

When bundled, we will retrieve it out of the `Resources` folder.
Locally, we're expected to run `script/download-rust-analyzer` and
put `vendor/bin` in our $PATH.

crates/workspace/src/lib.rs

@@ -327,7 +327,7 @@ impl Workspace {
                 params.languages.clone(),
                 params.client.clone(),
                 params.fs.clone(),
-                cx.background(),
+                cx,
             )
         });
         cx.observe(&project, |_, _, cx| cx.notify()).detach();