Production middlewares will only expose env vars that are statically
analyzable, as mentioned here: https://nextjs.org/docs/api-reference/next/server#how-do-i-access-environment-variables

This creates some incompatibility with `next dev` and `next start`,
where all `process.env` data is shared and can lead to unexpected
behavior in runtime.

This PR fixes it by limiting the data in `process.env` with the inferred
env vars from the code usage.

…nents too

Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>