CI: update actions/checkout to @v6

Author: linux_name2

.github/workflows/ci.yml

@@ -5,7 +5,7 @@ jobs:
   shellcheck:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - run: |
           shellcheck -s bash -e SC1090,SC1091,SC2010 \
             bin/atomic-upgrade \
@@ -18,14 +18,14 @@ jobs:
   python-lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - run: pip install --no-cache-dir ruff mypy
       - run: ruff check lib/atomic/fstab.py lib/atomic/rootdev.py
       - run: mypy --ignore-missing-imports lib/atomic/fstab.py lib/atomic/rootdev.py
 
   python-test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - run: pip install --no-cache-dir pytest
       - run: python -m pytest tests/ -v