firmware: in retrospect always fetch the signature file
firmware: in retrospect always fetch the signature file

When we move to the next major the file contents for
the txz are all the same but their signature is not.
So until the next changelog sync was carried out the
CHECKSUM matched and the signature will stay stale.

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core