Add Leap 15.6

Author: dcermak

.github/workflows/build-install.yaml

@@ -37,6 +37,7 @@ jobs:
           - 'registry.opensuse.org/opensuse/leap:15.3'
           - 'registry.opensuse.org/opensuse/leap:15.4'
           - 'registry.opensuse.org/opensuse/leap:15.5'
+          - 'registry.opensuse.org/opensuse/leap:15.6'
           - 'registry.opensuse.org/opensuse/tumbleweed'
 
           # CentOS Stream