Update coq-action.yml

Add Coq version 8.18 in CI testing.

workflow_dispatch allows one to manually trigger the CI test.

.github/workflows/coq-action.yml

@@ -1,6 +1,7 @@
 name: CI
 
 on:
+  workflow_dispatch
   push:
     branches: ['main']
   pull_request:
@@ -18,6 +19,7 @@ jobs:
           - '8.15'
           - '8.16'
           - '8.17'
+          - '8.18'
           - 'dev'
         ocaml_version:
           - 'default'