Bump actions/checkout from 4 to 6 (#5015)

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

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

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

.github/workflows/_nightly_image_build.yaml

@@ -19,7 +19,7 @@ jobs:
       image-tag: ${{ steps.build-image.outputs.image-tag }}
 
     steps:
-      - uses: actions/checkout@v6.0.1
+      - uses: actions/checkout@v6
 
       - name: Show build target
         run: |