build(deps): bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
     env:
       IMAGE: 'ghcr.io/ustclug/hackergame:latest'
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - name: Build Docker image

.github/workflows/smoketest.yml

@@ -16,7 +16,7 @@ jobs:
         # Debian 10 uses Python 3.7, which is not supported by Django 4.2 LTS
         python-version: [3.11, 3.9]
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
     - name: Set up Python ${{ matrix.python-version }}