Update to actions/checkout@v4. (#47)

This fixes CI warnings about Node.js 16 being deprecated.

.github/workflows/main.yml

@@ -24,7 +24,7 @@ jobs:
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
       with:
         submodules: recursive
 
@@ -50,7 +50,7 @@ jobs:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Install Rust
       run: rustup update nightly && rustup default nightly && rustup component add rustfmt
     - name: Format source code