Allow setting NODE_ENV for plugin

[IanVS]

Is your feature request related to a problem? Please describe.
I'm trying to find a way to run cypress locally in development against my own local endpoint as well as in netlify against my production endpoint. Normally, I could key off of process.env.NODE_ENV, but netlify does not set this value, and if I try to set it in [build.environment], then my development node modules are not installed and I cannot build my app.

Describe the solution you'd like
I'd like to have a way to set environment variables for only this plugin

Describe alternatives you've considered
I've looked through the docs to try to find if there's already a way to do this, but cannot find anything.