ci: lock down s390x CI to Ubuntu 20.04 runners

I am working on upgrading to 24.04 runners. In order to make sure that current jobs are scheduled
on Ubuntu 20.04, we need to ask for runners with tag `docker-main`, which is currently
set by those old runners.
Later, we will be able to switch this tag to `docker-noble-main` which are Ubuntu 24.04 runners.

Signed-off-by: Manu Bretelle <chantr4@gmail.com>

.github/workflows/test.yml

@@ -28,7 +28,7 @@ jobs:
             runs_on: ubuntu-20.04
             arch: 'x86_64'
           - kernel: 'LATEST'
-            runs_on: s390x
+            runs_on: ["s390x", "docker-main"]
             arch: 's390x'
     steps:
       - uses: actions/checkout@v4