clarify debuginfod extraction runs on worker, not in sbuild unshare

Clarify in the debuginfod blueprint that .debug extraction runs on the worker during artifact upload, not inside the sbuild unshare environment. The Requirements section previously said extraction must run "inside the isolated sbuild worker", which contradicted the Implementation plan that describes it as a helper called from _upload_binary_packages() / upload_artifact() after the build finishes. Rewords the Requirements bullet and the closing paragraph of the extraction section to match the implementation plan. Docs only; no code changes.

Closes: #1476 (closed)

Related: #957 (closed)