Use checkout action in version v3

The current version of actions/checkout is v3. That means that v2, which
we currently use, has been superseded. Update the version we use
accordingly.

Signed-off-by: Daniel Müller <deso@posteo.net>

.github/workflows/ondemand.yml

@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
     name: vmtest with customized pahole/Kernel
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ./.github/actions/setup
       - uses: ./.github/actions/vmtest
         with: