Remove hard coded production build from sapper build command

[johanalkstal]

sapper build is hard coded to set NODE_ENV to production.

This makes it impossible to do atleast two things,

1. Deploy an unoptimized/minified build of the application
2. Use the webpack config files to configure settings for different environments (using dotenv or Webpack.DefinePlugin or similar text replace tools) because process.env.NODE_ENV within the webpack files is always production.

I think a reasonable change is to have sapper build use whatever NODE_ENV is set to and if it doesn't exist, use production by default?