pkg-merge-updates: Do not hardcode Debian in the changelog

We may be merging from Apertis when being run on a dowstream
infrastructure.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

package-source-builder/overlay/usr/bin/apertis-pkg-merge-updates

@@ -133,7 +133,7 @@ def main():
     version = upstream_version.full_version + args.local_suffix + '0'
     bump_version(
         version,
-        [f'Sync from Debian {args.upstream}.'],
+        [f'Sync from {args.upstream}.'],
         release=True
     )
   git('commit', '--amend', '--no-edit', 'debian/changelog')