package-centric-testing: several improvements

This MR fixes several issues:

• Use APERTIS_RELEASE instead of the not defined RELEASE to clone TEST_REPO_URL.
• Use lava-runner templating to pass variables instead of hardcoding them in the LAVA yaml file with scripts.
• Use build-source job to retrieve debian source package instead of obs-capture.

Depends on tests/apertis-test-cases!591 (merged)

https://phabricator.apertis.org/T10253