osbuld-worker: call osbuild with --export flag

osbuild now supports using the `--export` flag (can be invoked multiple
times) to request the exporting of one or more artefacts.  Omitting it
causes the build job to export nothing.

The Koji API doesn't support the new image types (yet) so it simply uses
the "assembler" name, which is the final stage of the old (v1)
Manifests.

internal/weldr/api.go

@@ -256,6 +256,7 @@ func (api *API) getComposeStatus(compose store.Compose) *composeStatus {
 	if err != nil {
 		panic(err)
 	}
+
 	return &composeStatus{
 		State:    composeStateFromJobStatus(jobStatus, &result),
 		Queued:   jobStatus.Queued,
@@ -2024,6 +2025,7 @@ func (api *API) composeHandler(writer http.ResponseWriter, request *http.Request
 			Targets:         targets,
 			ImageName:       imageType.Filename(),
 			StreamOptimized: imageType.Name() == "vmdk", // https://github.com/osbuild/osbuild/issues/528
+			Exports:         imageType.Exports(),
 		})
 		if err == nil {
 			err = api.store.PushCompose(composeID, manifest, imageType, bp, size, targets, jobId)