flatdeb-builder: Fix test by not hardcoding the release name

Rather than hardcoding the release name in the filename of the YAML definition of the APT suite, generate it at runtime so that we can also inherit the osname and APT mirror from the main settings, which is useful for downstreams.

flatdeb-builder/suites/default.yaml.tpl

@@ -3,8 +3,8 @@ can_merge_usr: true
 has_automatic_dbgsym: true
 strip_source_version_suffix: bv20\d\d.*b\d+|b\w\d+
 sources:
-  - label: apertis-target
-    apt_uri: 'https://repositories.apertis.org/apertis/'
+  - label: %DISTRO%-target
+    apt_uri: '%MIRROR%'
     apt_components:
       - target
 ...