Update the CI configuration (#65)

1. Avoid running duplicate checks for pull requests.
2. Bump actions/checkout to v4.
3. Add a formatting check, using rustfmt.

.github/workflows/ci.yml

@@ -1,6 +1,10 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   build:
@@ -12,7 +16,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
       - name: Install Rust
         uses: actions-rs/toolchain@v1
         with:
@@ -22,3 +26,14 @@ jobs:
         run: cargo build --verbose
       - name: Test the code
         run: cargo test --verbose
+  check-formatting:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust
+        uses: actions-rs/toolchain@v1
+        with:
+            toolchain: stable
+            override: true
+      - name: Check formatting
+        run: cargo fmt --check