Support `NPM_CONFIG_USERCONFIG` environment variable

[r-thomson]

Is your enhancement request related to a problem? Please describe.
I ran into a situation recently where I wanted to use a Heroku-specific .npmrc file. My initial thought was to create a second file (.heroku.npmrc) and point NPM at it by setting the environment variable NPM_CONFIG_USERCONFIG=.heroku.npmrc. However, npm ci is run with the --userconfig flag set, which overrides that environment variable (introduced in #79).

Describe the solution you'd like
It'd be nice if the --userconfig flag was skipped if the NPM_CONFIG_USERCONFIG environment variable is present. Or, the buildpack could be changed to set NPM_CONFIG_USERCONFIG (if it's unset) instead of using the --userconfig flag.

Describe alternatives you've considered
As a workaround, I'm using a prebuild script to overwrite .npmrc with the contents of .heroku.npmrc.