Bump actions/checkout from 4 to 6 (#4380)

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

- vLLM main:
2918c1b49c

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

.github/workflows/_e2e_nightly_multi_node.yaml

@@ -106,7 +106,7 @@ jobs:
             echo "${{ secrets.KUBECONFIG_B64 }}" | base64 -d > $KUBECONFIG
 
         - name: Checkout code
-          uses: actions/checkout@v4
+          uses: actions/checkout@v6
 
         - name: Prepare scripts
           run: |