Update CI configuration for salsa-ci switch to sbuild
Backport !1548 (merged) and !1645 (merged) to the debian/5.10/bullseye-security branch:
- d/salsa-ci.yml: Merge the extract-source job into the build's job script
- d/b/gencontrol.py: Extend the effect of
$DEBIAN_KERNEL_DISABLE_INSTALLER
- d/salsa-ci.yml: Early move orig tarballs back where they can be cached