[CI] Bump actions/checkout from 4 to 6 (#6255)

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.

- vLLM version: v0.14.1
- vLLM main:
d68209402d

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/schedule_lint_image_build.yaml

@@ -22,7 +22,7 @@ jobs:
     name: vllm-ascend lint image build
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
         persist-credentials: false