Fix kernel_config_url rc value

Since 5a91f8f2, an extra /artifacts/ was accidentally removed from the value of 'kernel_config_url'.

To avoid hardcoding artifacts/ before the artifacts_url call, change NEW_CONFIG_NAME to contain artifacts/ as a prefix, same as CONFIG_FILE already does: https://gitlab.com/cki-project/pipeline-definition/-/blob/60ceafa841f8d23f69531a3febb9a4f8544c4390/cki_pipeline.yml#L1925