Bug report

Describe the bug

https://github.com/zeit/next.js/blob/cffa2e3f2165b58b77fb354d3f74a4ac8222b68f/packages/next/types/global.d.ts#L1-L12

↑
This file overrides the original implementation of the ProcessEnv typing, which causes negative side effects in third-party libraries that rely on it. In my case this was execa.

To Reproduce

1. Create a Next.js app, ensure tsconfig.json and next-env.d.ts have been autogenerated.

2. Install execa and ts-node.

   yarn add execa ts-node
   

3. In test.ts write something like:

   import execa from "execa";
   
   execa("printenv", [], {
       env: {
         HELLO: "world",
         NODE_ENV: "development",
       },
       extendEnv: false,
       stdout: "inherit",
     });

4. Observe no typing issues and ensure that the command works by running:

   yarn ts-node --transpile-only  --compiler-options="{\"module\": \"commonjs\"}" test.ts
   

   Output:

   HELLO=world
   NODE_ENV=development
   ✨  Done in 0.42s.
   

5. Comment out NODE_ENV: "development", in test.ts, which is a totally valid action.

6. Observe the following TypeScript error in the editor:

   Argument of type '{ env: { HELLO: string; }; extendEnv: false; stdout: "inherit"; }' is not assignable to parameter of type 'Options<null>'.
     Types of property 'env' are incompatible.
       Property 'NODE_ENV' is missing in type '{ HELLO: string; }' but required in type 'ProcessEnv'.ts(2345)
   global.d.ts(10, 14): 'NODE_ENV' is declared here.
   

Expected behavior

I would not expect Next.js to pollute Node's ProcessEnv interface, thus not causing typing false-positives in third-party libraries like execa. I'm using execa in a project that includes a few data-processing commands in addition to the Next.js app itself.

System information

• Version of Next.js: 9.0.3

Additional context

git blame points to #7648