Infra: Make package.json scripts work on windows.

[ryan-roemer]

We presently have things like eval (yarn -s env) to place environment variables for usage in downstream commands. This won't work on windows.

Tasks:

• Come up with windows friendly package.json scripts
• With overridable environment variables (e.g., STAGE=sandbox)
• That also has default values (e.g. ${AWS_REGION:-us-east-1})

Some ideas:

• cross-env (but doesn't support default values)
• builder (but kind of a pain since env support is all big json strings and you have to invoke builder CLI
• A custom Node.js script to provide defaults and take in options easily in whatever format we want.