Fix mgr-sign-metadata-ctl check-channels when checking for signatures in repomd metadata (bsc#1233884)

[mcalmer]

What does this PR change?

The mgr-sign-metadata-ctl check-channels command is currently only checking for debian repos correctly.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: cmd tool no tests

• DONE

Links

Port(s): TBD

• DONE

Changelogs

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