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we currently use, has been superseded.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/actions/vmtest/action.yml

@@ -16,7 +16,7 @@ inputs:
 runs:
   using: "composite"
   steps:
-    # setup envinronment
+    # setup environment
     - name: Setup environment
       uses: libbpf/ci/setup-build-env@master
       with: