Allow manual dispatch of CI on github.

Author: cessen

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ on:
     # Every-other month, to make sure new Rust
     # releases don't break things.
     - cron: '15 12 5 1,3,5,7,9,11 *'
+  workflow_dispatch:
 
 env:
   CARGO_TERM_COLOR: always