Bump actions/checkout from 4 to 6 (#5234)

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

- vLLM version: release/v0.13.0
- 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/_pr_image_build.yaml

@@ -38,7 +38,7 @@ jobs:
             runner: ubuntu-22.04-arm
             tag: arm64
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
         persist-credentials: false
@@ -100,7 +100,7 @@ jobs:
     if: ${{ inputs.should_push }}
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Download arm64 digests
         uses: actions/download-artifact@v4