ci/packit: set downstream name

Since our upstream repository name differs from the downstream name we
should set the `downstream_package_name` [1] config option.

[1]: https://packit.dev/docs/configuration#downstream_package_name

Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

.packit.yaml

@@ -1,6 +1,7 @@
 # https://packit.dev/docs/configuration/
 
 specfile_path: image-builder.spec
+downstream_package_name: image-builder
 
 files_to_sync:
   - image-builder.spec