reproducible_debian_list_build: The origin of the d-i installer can now

be specified.

For testing and unstable the d-i installer will be built from git.
Note for later: As soon as bookworm enters the (deep) freeze, the d-i
installer should not be built from git any more.

The new feature was introduced in
live-team/live-build!293



Signed-off-by: Holger Levsen <holger@layer-acht.org>