Update actions/checkout action to v4

Author: taiki-e

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
     branches:
       - master
   schedule:
-    - cron: "0 1 * * *"
+    - cron: "0 1 * * 0"
 
 env:
   CARGO_INCREMENTAL: 0
@@ -38,7 +38,7 @@ jobs:
           - nightly
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
       - run: cargo build
@@ -48,23 +48,23 @@ jobs:
   clippy:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update stable
       - run: cargo clippy --all --all-targets
 
   rustfmt:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update stable
       - run: cargo fmt --all --check
 
   rustdoc:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Rust
         run: rustup update nightly && rustup default nightly
       - run: cargo doc --no-deps