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we currently use, has been superseded.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/test.yml

@@ -31,7 +31,7 @@ jobs:
             runs_on: z15
             arch: 's390x'
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
         name: Checkout
       - uses: ./.github/actions/setup
         name: Setup