Comment # 2 on bug 1227304 from Adrian Schröter

yes, it is too late to do that at build time. The VM has no network and the
dependencies can not pre-calculated.

Either do that in default mode, so the change gets commited to the source.

Alternative is to use dynamic build dependency implementation where another
build job gets started in case dependencies are changing. That is a generic rpm
feature, supported by OBS by respawning a new job.
https://fedoraproject.org/wiki/Changes/DynamicBuildRequires


closing as works as designed.