Detection of duplicate Network Lock signature chains added in
01847e0123 failed to account for chains
originating with a SigCredential signature, which is used for wrapped
auth keys. This results in erroneous removal of signatures that
originate from the same re-usable auth key.
This change ensures that multiple nodes created by the same re-usable
auth key are not getting filtered out by the network lock.
Updates tailscale/corp#19764
Signed-off-by: Anton Tolchanov <anton@tailscale.com>