debian/salsa-ci.yml: enable salsa-ci

debian/salsa-ci.yml: enable salsa-ci

according to the manual at
https://salsa.debian.org/salsa-ci-team/pipeline/-/blob/master/README.md

If the pipeline is not yet enabled and configured in the gitlab web
interface by default, this needs to be done manually as written in the
manual.

As of now the salsa-ci only supports gbp, so convert the debrebase
repository to a gbp one.
Also work around a problem with gbp export-orig replacing variables in
.gitarchive-info which then causes problems as the repository and the
generated .tar.gz don't match.

Allow reprotest to fail for now as the build is currently not
reproducible.

Disable blhc as xen currently does not enable hardening when building
the hypervisor so blhc outputs many errors.

Signed-off-by: Maximilian Engelhardt <maxi@daemonizer.de>
Acked-by: Hans van Kranenburg <hans@knorrie.org>