cockroach-admin-server: use a smaller tokio worker pool (#8599)

followup from
#8555 (comment)

Author: jgallagher

cockroach-admin/src/bin/cockroach-admin.rs

@@ -51,7 +51,9 @@ enum Args {
 }
 
 fn main() {
-    if let Err(err) = oxide_tokio_rt::run(main_impl()) {
+    let mut builder = oxide_tokio_rt::Builder::new_multi_thread();
+    builder.worker_threads(8);
+    if let Err(err) = oxide_tokio_rt::run_builder(&mut builder, main_impl()) {
         fatal(err);
     }
 }