ci-package-builder: Build sources only once

Simplify the pipeline by:
1. computing if this is a release build in the preparation stage
2. building the sources only once
3. avoiding the _build-{snapshot|release} directory dance

This saves some resources: on big packages building the sources can
take a non-trivial amount of time and doing it twice in a single run is
not the greatest idea ever.

For instance, `texlive-extra` needs more than 30 minutes for each
*source* build and with this we build it only once.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>