ci: bump checkout action to v4

Due to the transition from Node 16 to Node 20, the checkout action
needs to be updated to v4.

More info: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

Signed-off-by: Geyslan Gregório <geyslan@gmail.com>

.github/workflows/ondemand.yml

@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
     name: vmtest with customized pahole/Kernel
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ./.github/actions/setup
       - uses: ./.github/actions/vmtest
         with: