build: Avoid having to steps when not using the snapshot

build: Avoid having to steps when not using the snapshot

The build environment can be cached in a snapshot which allows much
faster builds. But sometimes, we don't want to use the snapshot.

In those cases, we will install the default set of packages first and
then we will install the source package. In order to find any dependency
problems quicker, this is now being done in just one step.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>