Set $NPM_REBUILD to rebuild cached deps

[mojodna]

When composed with other buildpacks, underlying dependencies (e.g. mapnik may have changed causing the cached binaries to link to incorrect versions. If prior buildpacks (or users) set $NPM_REBUILD, cached modules will be rebuilt.

[zeke]

@mojodna are you using this with user-env-compile enabled? We are in the process of making user-env-compile the default for all apps. This effectively means config vars will be available to the buildpacks compile script as environment variables.

[mojodna]

Yup. This means that other buildpacks can manipulate config vars?

The same use-case applies, so let me know if I should update the pull.

[zeke]

This means other buildpacks can read config vars, but not manipulate them. (Thought buildpacks can manipulate the environment with a .profile.d script.)

If you don't mind, let's leave this PR open until user-env-compile becomes the default for all apps.

[mojodna]

Is there a blessed way to pass environment vars between buildpacks (e.g. LIBRARY_PATH, etc.)? ddollar/heroku-buildpack-multi#18, essentially.

[zeke]

Hey @mojodna, it's now possible to access config vars during build without using user-env-compile (which is now dead. Yay!). Does this change in this PR still reflect a need that you have?

[mojodna]

Not recently, no. I'll close.