[9.20] fix: usr: Fix dnssec-keygen key collision checking for KEY rrtype keys

The :iscman:`dnssec-keygen` utility program failed to detect
possible Key ID collisions with the existing keys generated
using the non-default ``-T KEY`` option (e.g. for ``SIG(0)``).
This has been fixed.

Closes #5506

Backport of MR !11047

Merge branch 'backport-5506-dnssec-keygen-sig0-keys-collision-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11128