Pass .env file variables to getPlatformProxy

[james-elicx]

Wrangler's getPlatformProxy() does not read variables from the .env file. This is problematic as we are asking people to use a utility in a Node.js dev server that expects people to use a .env file, instead of Wrangler's custom way of reading env vars.

We should look at reading the variables defined in .env files in next-on-pages' wrapper around getPlatformProxy(), and forward these env vars to the call to getPlatformProxy().