apertis-pkg-merge-*: Make the local suffix customizable

Downstreams need a different local suffix for their packages and we may change ours at some point.

.gitlab-ci.yml

@@ -149,7 +149,7 @@ test-package-source-builder:
     - rm -rf fixture-dash-unmodified
     - git clone https://gitlab.apertis.org/infrastructure/test-data/fixture-dash-unmodified
     - ": Verify we correctly handle custom local suffixes"
-    - cd fixture-dash-modified
+    - cd fixture-dash-unmodified
     - apertis-pkg-merge-updates --package dash --upstream debian/buster --downstream apertis/v2020dev0 --local-version-suffix aper
     - git log --graph --oneline --decorate
     - git diff debian/buster..