Cleanup jobs requesting an empty OS at the job classad

[amaltaro]

Impact of the bug
WMAgent

Describe the bug
While validating these job wrapper changes:
#11077

I noticed that Cleanup jobs actually define an empty string for the required OS:

  CMS_JobType = "Cleanup"
  REQUIRED_OS = ""

How to reproduce it
Create a workflow and check the job classads for a Cleanup job.

Expected behavior
Cleanup jobs don't rely on any CMSSW release, so ScramArch is actually not defined for this step.
When converting the job/step requirements into job classads, we should make sure that REQUIRED_OS has a correct value, which is either any or the rhelX string.

Additional context and error message
None