Use a pre-built executable as the reference named

Use a pre-built executable as the reference named

The "respdiff" GitLab CI job compares DNS responses produced by the
current version of named with those produced by a reference version.
The latter is built from source in each "respdiff" job, despite the fact
that the reference version changes very rarely.  Use a pre-built named
executable as the reference version instead, assuming it is available in
the OS image used for "respdiff" tests.