In standalone mode, `server.js` hardcoded NODE_ENV as production

Link to the code that reproduces this issue

https://github.com/HugoDerigny/standalone-production-env-bug

To Reproduce

From above repo

1. Run npm run build
2. See server.js output file
3. Run node .next/standalone/server.js
4. See the logs that says it's production environment

From empty nextjs repo

1. Create Next App
2. Configure next.config.js output to be standalone
3. Set NODE_ENV as test on your computer: export NODE_ENV=test
4. Create a .env.test file
5. Run next build
   -> You will see that the build take envs from .env.test.

$ next build
   ▲ Next.js 14.0.2
   - Environments: .env.test

BUT

In build files : .next > standalone > server.js you can see that NODE_ENV is hardcoded :

/** l.5 */ process.env.NODE_ENV = 'production'

Current vs. Expected behavior

Current behavior:
NODE_ENV is hardcoded as production

Expected behavior
server.js must not redefine NODE_ENV if it's already defined as test since it's the only other possible value.

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:43 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T6000
Binaries:
  Node: 18.17.1
  npm: 9.6.7
  Yarn: 1.22.19
  pnpm: 7.29.1
Relevant Packages:
  next: 14.0.3-canary.0
  eslint-config-next: 14.0.2
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.2.2
Next.js Config:
  output: standalon

Which area(s) are affected? (Select all that apply)

Standalone mode (output: "standalone")

Additional context

Even when using next build if you console.log(process.env.NODE_ENV) in the root layout.tsx, it logs production in the build trace.

Start with standalone output generation and the generated .next/standalone/server.js, using the linked reproduction or the listed next build steps with output: "standalone". Trace how NODE_ENV is assigned and inspect nearby standalone tests, then verify that an existing NODE_ENV is preserved and the generated server behaves as expected.