chore(ci): run Rust CI on workflow_dispatch (#292)

This should fix the Rust CI not being run after approving the workflows
for new contributors.

Author: m4tx

.github/workflows/rust.yml

@@ -5,6 +5,7 @@ on:
   pull_request:
   schedule:
     - cron: "0 0 * * *"
+  workflow_dispatch:
 
 env:
   CARGO_TERM_COLOR: always