result:
after changing our BUILDPACK_URL in the heroku.yml, our heroku app was still using autodetect.

expected result:
heroku_san should either update the heroku configuration, or provide a warning message that the BUILDPACK_URL does not match the application, when deploying.

our heroku.yml:
production:
app: cases-render-service-prod
config:
NODE_ENV: production
BUILDPACK_URL: https://github.com/ddollar/heroku-buildpack-multi.git
staging:
app: cases-render-service-staging
config:
NODE_ENV: production
BUILDPACK_URL: https://github.com/ddollar/heroku-buildpack-multi.git