release: Don't break if firmware install fails

On some platforms (e.g. powerpc) we don't have packages, so we can't
install them onto the ISOs. Proceed with building the images anyway.

Reported by: Weekly snapshot builds
Fixes: 7e2996c1f5b4 ("release: install wireless firmware onto disc1 and dvd")
MFC after: 1 minute

(cherry picked from commit e8263ace39c8ecf11233c0a10d0b1839e6813046)