Revert "CI: disable opensuse mkosi CI"

This reverts commit ab6df52.

The image build failed during kernel RPM installation (bug in %post
scriptlet). This has been fixed in the package suse-module-tools 16.0.13
[1]. The fix is in openSUSE Tumbleweed repos so the tests can be enabled
again.

[1] openSUSE/suse-module-tools#53

Fixes: #21019

.github/workflows/mkosi.yml

@@ -23,8 +23,7 @@ jobs:
           - debian
           - ubuntu
           - fedora
-          # Disabled until https://github.com/systemd/systemd/issues/21019 is fixed.
-          # - opensuse
+          - opensuse
 
     steps:
     - uses: actions/checkout@v2