Prevent building libmemcached on stacks that don't vendor it (#964)
Prevent building libmemcached on stacks that don't vendor it (#964)

The `libmemcached` package is available in the base stack image for all
stacks newer than `cedar-14`, so at buildpack compile time the vendor
step is skipped for those stacks:
https://github.com/heroku/heroku-buildpack-python/blob/106f2997fa124852a2a35ee8bfa604ad20c47988/bin/steps/pylibmc#L12-L15

As such, it is not necessary to run the libmemcached bob-builder formula
on newer stacks. The conditional has been updated so it correctly handles
heroku-18 and also the upcoming heroku-20.

An exit code of 1 has been used, otherwise `bob upload` will build and
then upload a zero byte archive to S3, which will go unused.

(This is in comparison to bob formulas that are nested, where an exit
code of 0 is actually desirable, since it allows skipping steps.)

Refs W-7485877.

Co-authored-by: Joe Kutner <a7c646dd5a3c773c65c611f90c1059aee61c7d07@gmail.com>
heroku/heroku-buildpack-python,Scalingo/python-buildpack,Scalingo/python-buildpack,skilljar/heroku-buildpack-python,AltSchool/heroku-buildpack-python,AltSchool/heroku-buildpack-python,heroku/heroku-buildpack-python