It turns out that upstream builder needs to be run as root, otherwise they can't work:
dcaro@toolsbeta-sgebastion-05:~$ kubectl --as=admin --as-group=system:masters -n image-build logs test-buildpacks-pipelinerun-nsxt5-build-from-git-pod -c step-build Warning: Platform requested deprecated API '0.4' Warning: Buildpack 'heroku/python@0.0.0' requests deprecated API '0.4' mkdir: cannot create directory ‘.heroku’: Permission denied ERROR: failed to build: exit status 1