Do not enter or test SCC credentials when we use a mirror

[mcalmer]

What does this PR change?

Port of https://github.com/SUSE/spacewalk/pull/22634

Required when #7593 gets merged

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: already covered

• DONE

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