Only run prepublish script on actual npm publish

[mourner]

Makes prepublish script run only when executed in context of npm publish, addressing previous concerns with #3476. We'll get rid of it after setting up publishing from Circle. @jfirebaugh @lucaswoj

[andrewharvey]

cc @jfirebaugh @mourner @lucaswoj

I'm using yarn add https://github.com/mapbox/mapbox-gl-js.git#master in my project to use the master branch, however this PR applied the in-published check in prepublish. That means that when mapbox-gl get's installed automatically when I yarn or npm install my project it doesn't build the mapbox-gl dist files, but the dist files are needed for both the stylesheet and the main defined in the package.json, so the package doesn't work.

Does this still make sense now that the dist files are used as the entry point (done in #4423)?