Do not re-assign process.env

Re-assigning `process.env` substitutes the "magic object" that sets
environment variables at the process level with an ordinary JavaScript
object. This causes environment variables that are set after
`process.env` is re-assigned to not be visible to native add-ons.

See [this Node.js issue][issue] and [this reproduction case][repro]
for details.

[issue]: nodejs/node#46996
[repro]: https://github.com/unflxw/nodejs-process-env-addons-repro

packages/next-env/index.ts

@@ -20,6 +20,18 @@ type Log = {
   error: (...args: any[]) => void
 }
 
+function replaceProcessEnv(sourceEnv: Env) {
+  Object.keys(process.env).forEach((key) => {
+    if (sourceEnv[key] === undefined || sourceEnv[key] === '') {
+      delete process.env[key]
+    }
+  })
+
+  Object.entries(sourceEnv).forEach(([key, value]) => {
+    process.env[key] = value
+  })
+}
+
 export function processEnv(
   loadedEnvFiles: LoadedEnvFiles,
   dir?: string,
@@ -94,7 +106,7 @@ export function loadEnvConfig(
   if (combinedEnv && !forceReload) {
     return { combinedEnv, loadedEnvFiles: cachedLoadedEnvFiles }
   }
-  process.env = Object.assign({}, initialEnv)
+  replaceProcessEnv(initialEnv)
   previousLoadedEnvFiles = cachedLoadedEnvFiles
   cachedLoadedEnvFiles = []
 

test/unit/preserve-process-env.test.ts

@@ -0,0 +1,9 @@
+import { loadEnvConfig } from '../../packages/next-env/'
+
+describe('preserve process env', () => {
+  it('should not reassign `process.env`', () => {
+    const originalProcessEnv = process.env
+    loadEnvConfig('.')
+    expect(Object.is(originalProcessEnv, process.env)).toBeTrue()
+  })
+})