bypass detect step

[jromero]

Cloned from https://github.com/tektoncd/catalog/issues/161

Expected Behavior

There should be a way for a user to set the desired buildpack and skip the detection work. I wouldn't expect the Tekton step as a whole to be skipped, however I would expect it to take in my input and make it a no-op.

The pack CLI has the option to do this when building images:

pack build --buildpack

Actual Behavior

Steps to Reproduce the Problem

Additional Info