env is hardcoded to "production" which causes React v17 to fail building properly

Current behavior

I think there's a mismatch of environment variables being read by babel (JSX config) and Styleguidist, causing some issues deeper down.

(I've noticed this was raised before but was closed)

• build.js sets the env to be production here: https://github.com/styleguidist/react-styleguidist/blob/master/src/scripts/build.ts#L9
• This check is redundant because it will always be true, as env is hardcoded from the call above https://github.com/styleguidist/react-styleguidist/blob/master/src/scripts/make-webpack-config.ts#L31
• Again, env is hardcoded to production and then passed through here https://github.com/styleguidist/react-styleguidist/blob/master/src/scripts/make-webpack-config.ts#L131

So the issue is. My webpack config has mode set to "production" (because mode comes from env) and anything else that uses NODE_ENV will see developement.

I ended up with this error: ReferenceError: _jsx is not defined.

I don't know why its hardcoded to production regardless of what you set, but it seems to be the cause of this bug.

Potential Fix https://github.com/styleguidist/react-styleguidist/blob/master/src/scripts/make-webpack-config.ts#L29 should be

env = process.env.NODE_ENV || env

https://github.com/styleguidist/react-styleguidist/blob/master/src/scripts/build.ts#L9 should also default to 'development' instead of 'production'