ci: enable running the pipeline manually using workflow_dispatch

Pipelines with `on: workflow_dispatch` can be executed using a button in
the web interface or the API. This can come in handy every then and now.

Signed-off-by: Leonard Göhrs <leonard@goehrs.eu>

.github/workflows/rust.yml

@@ -7,6 +7,7 @@ on:
     branches: [ "main" ]
   schedule:
     - cron: '34 11 * * 2'
+  workflow_dispatch:
 
 env:
   CARGO_TERM_COLOR: always