fix(ci): run CI on pull request events (#14)

This commit fixes the GitHub Actions workflow file so that it will be run not only on the push event on the main branch but also pull request events.

.github/workflows/ci.yml

@@ -1,15 +1,21 @@
 name: CI
 
-on: push
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
 
 jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
-      - uses: goto-bus-stop/setup-zig@v1
+      - uses: actions/checkout@v4
+      - uses: goto-bus-stop/setup-zig@v2
         with:
-          version: master
+          version: 0.12.0
       - name: fmt
         run: zig fmt --check *.zig src/*.zig
       - name: test