Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
     name: Test examples
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install toolchain
         uses: actions-rs/toolchain@v1
         with: