component/re6stnet: correctly extend stack/slapos.cfg

Without this fix, we cannot compile re6st-node because we don't use the
correct version of setuptools.
2 jobs for bgp
Status Job ID Name Coverage
  External
failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master