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/coverity.yml

@@ -11,7 +11,7 @@ jobs:
     if: github.repository == 'libbpf/libbpf'
     name: Coverity
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ./.github/actions/setup
       - name: Run coverity
         run: |