Build source package only once

Instead of letting sbuild create a source package every time this creates it only once and passes it through the artifacts. This means that the build source job needs to finish before the other build jobs can start. This makes it closer to how the Debian archive works where a maintainer uploads the source package and the buildds compile a dsc. Also note that most jobs can now run without a git clone (GIT_STRATEGY: empty).

• Drop the BUILD_DIR variable as it is unused with sbuild.
• Use dpkg-buildpackage instead of gbp buildpackage to create a source package as it is enough and works in more cases.