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/project_panel/src/lib.rs

@@ -622,7 +622,7 @@ mod tests {
                 params.languages.clone(),
                 params.client.clone(),
                 params.fs.clone(),
-                cx.background(),
+                cx,
             )
         });
         let root1 = project