Run s390x tests on Ubuntu 20.04

Signed-off-by: Stefan Scherer <stefan.scherer@docker.com>

Jenkinsfile

@@ -558,7 +558,7 @@ pipeline {
                             expression { params.s390x }
                         }
                     }
-                    agent { label 's390x-ubuntu-1804' }
+                    agent { label 's390x-ubuntu-2004' }
 
                     stages {
                         stage("Print info") {
@@ -667,7 +667,7 @@ pipeline {
                         not { changeRequest() }
                         expression { params.s390x }
                     }
-                    agent { label 's390x-ubuntu-1804' }
+                    agent { label 's390x-ubuntu-2004' }
 
                     stages {
                         stage("Print info") {