Allow using system node (#18172)

Release Notes:

- (Potentially breaking change) Zed will now use the node installed on
your $PATH (if it is more recent than v18) instead of downloading its
own. You can disable the new behavior with `{"node":
{"disable_path_lookup": true}}` in your settings. We do not yet use
system/project-local node_modules.

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/zed/src/zed.rs

@@ -3365,7 +3365,7 @@ mod tests {
         cx.set_global(settings);
         let languages = LanguageRegistry::test(cx.executor());
         let languages = Arc::new(languages);
-        let node_runtime = node_runtime::FakeNodeRuntime::new();
+        let node_runtime = node_runtime::NodeRuntime::unavailable();
         cx.update(|cx| {
             languages::init(languages.clone(), node_runtime, cx);
         });