Refine workflow triggers

This workflow will run on:

- pushes to main,
- pull requests against main, and
- by manual invocation via GitHub's UI

.github/workflows/ci.yml

@@ -1,9 +1,12 @@
 name: CI
+
 on:
-  pull_request:
   push:
-    branches:
-      - main
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+  workflow_dispatch:
+
 jobs:
   test:
     runs-on: ubuntu-20.04