Update actions/checkout action to v4 (#251)

Author: taiki-e

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
           - aarch64-unknown-linux-gnu
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions-rs/cargo@v1
         with:
           command: test
@@ -40,7 +40,7 @@ jobs:
           - x86_64-unknown-linux-gnu
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: nightly
@@ -69,7 +69,7 @@ jobs:
           - aarch64-unknown-linux-gnu
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions-rs/toolchain@v1
         with:
             toolchain: ${{ matrix.toolchain }}
@@ -85,7 +85,7 @@ jobs:
     name: fmt
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update stable
       - name: Install rustfmt