Use other nodejs verson as defined via npm-default

Dear all! I'm trying to build a nodejs app https://build.opensuse.org/package/show/devel:kubic/traefik that's UI is build on the OBS using the "local-npm-registry". This pulls "npm-default" which is version 21 on Tumbleweed. Unfortunately the package can be built only with sable versions 16, 18, 20 but not with the (current) default 21 version. Is there a way to configure the spec file to install and use npm20 instead of npm21? Best regards, Johannes