Do not restore cache when using npm ci

With the current implementation of `npm ci` usage too create the build environment and Heroku slug, the buildpack will restore the cache at the beginning of the build and save the cache at the end of the build.

As recommended by the npm docs (https://docs.npmjs.com/cli/v6/commands/npm-ci#description), there should be no `node_modules` before an `npm ci` run because the command will delete the `node_modules` anyways. (The Heroku buildpack will pull `node_modules` from the previous build and restore them to the node_modules directory if the build is configured to use `NODE_MODULES_CACHE`.)

Start by tracing the buildpack's cache restore and save paths for builds using npm ci and NODE_MODULES_CACHE. Done means an npm ci build does not restore node_modules before npm ci runs, while the intended cache behavior is preserved for other builds.

Written by the indexing model from the issue text.