I ran into the same issue (RC1 also didn’t have some memory fixes that were done in 0.28.5, so we needed it rather urgently).
What I’ve done is fork the buildpack and changed the version, so as a workaround (for now), you can change the buildpack on heroku to not be https://github.com/metabase/metabase-buildpack but use: https://github.com/pjaspers/metabase-buildpack