Build unit tests in the unit test job

Build unit tests in the unit test job

Building the unit tests in the build job ships them in the CI artifact
(+200 MB) and transfers them over the network.  Build them in the unit
test job instead.

When Git checks out the sources, their modification times are newer than
the build tree restored from the artifact, so meson would rebuild all of
BIND 9 in the unit test job.  Age the tracked sources so the build is
treated as up to date and only the unit tests get compiled.

Assisted-by: Claude:claude-opus-4-8