ci: simplify syntax for "on:"

ci: add "workflow_syntax" in "on" for "main" branch

.github/workflows/ci.yml

@@ -4,12 +4,11 @@ env:
   DOTENV_CONFIG_PATH: "./.env.example"
 
 on:
+  workflow_dispatch:
   pull_request:
-    branches:
-      - "main"
   push:
     branches:
-      - "main"
+      - main
 
 jobs:
   ci: