Tests: Run integration tests on SCOS images whenever available

[bgartzi]

SCOS images do not support secure boot yet. CentOS 9 and 10 images started supporting it starting on the 20250825 update:

https://lists.centos.org/hyperkitty/list/devel@lists.centos.org/thread/CC4HSMFZG3BLMRNLNVIULGTV3TQKHWQE/

However, scos images are not there yet, as the latest available images are older:

• https://cloud.centos.org/centos/scos/9/prod/streams/
• https://cloud.centos.org/centos/scos/10/prod/streams/

#24 was merged without testing scos because of that.

We need to add scos to the test matrix as soon as images supporting secure boot are published.