Ensure npm install and prune aren't affected by `NODE_ENV`

[colincasey]

Right now, during this CNB's build phase, we should guarantee that:

• all dependencies will be installed before supported scripts in package.json are executed (i.e.; heroku-prebuild, heroku-build or build, heroku-postbuild)
• dev dependencies are pruned at the end of the CNB build phase so that production images don't contain unnecessary packages

But, with the introduction of npm dev dependency pruning in #1126, the install and prune commands aren't fully isolated from NODE_ENV if it is set which can produce unexpected results (see #1126 (comment)).