ci: Allow manually invoking tests

I want to be able to run them manually too to sanity check things.
https://docs.github.com/en/actions/using-workflows/triggering-a-workflow#defining-inputs-for-manually-triggered-workflows

.github/workflows/rust.yml

@@ -11,6 +11,7 @@ on:
     branches: [main]
   pull_request:
     branches: [main]
+  workflow_dispatch: {}
 
 env:
   CARGO_TERM_COLOR: always