fix X-Instance-Identifier header send when doing a product refresh at Cloud RMT Server

[mcalmer]

What does this PR change?

Doing product refresh against RMT Cloud infrastructure for PAYG images fail as we send the wrong
Header data.
We need to send the data from the base product and not the data from /etc/os-release

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

• No tests: already covered

• DONE

Links

Port of https://github.com/SUSE/spacewalk/pull/22441

• DONE

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