CI: Update actions/checkout to v4

Author: rnestler

.github/workflows/ci.yml

@@ -10,22 +10,22 @@ jobs:
     runs-on: ubuntu-22.04
     container: rust:1.60
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: cargo build
       - run: cargo test
 
   lint:
     runs-on: ubuntu-22.04
     container: rust:1.78
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: rustup component add clippy
       - run: cargo clippy --all-targets --all-features -- -D warnings
 
   fmt:
     runs-on: ubuntu-22.04
     container: rust:1.78
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: rustup component add rustfmt
       - run: cargo fmt -- --check