chg: ci: 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.

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

Assisted-by: Claude:claude-opus-4-8
Merge branch 'mnowak/build-unit-tests-in-unit-job' into 'main'

See merge request isc-projects/bind9!12180