Clarify .env.local loading when NODE_ENV=test (#22139)

docs/guides/runtime/set-env.md

@@ -17,7 +17,7 @@ Bun reads the following files automatically (listed in order of increasing prece
 
 - `.env`
 - `.env.production`, `.env.development`, `.env.test` (depending on value of `NODE_ENV`)
-- `.env.local`
+- `.env.local` (not loaded when `NODE_ENV=test`)
 
 ```txt#.env
 FOO=hello

docs/runtime/env.md

@@ -8,6 +8,10 @@ Bun reads the following files automatically (listed in order of increasing prece
 - `.env.production`, `.env.development`, `.env.test` (depending on value of `NODE_ENV`)
 - `.env.local`
 
+{% callout %}
+**Note:** When `NODE_ENV=test`, `.env.local` is **not** loaded. This ensures consistent test environments across different executions by preventing local overrides during testing. This behavior matches popular frameworks like [Next.js](https://nextjs.org/docs/pages/guides/environment-variables#test-environment-variables) and [Create React App](https://create-react-app.dev/docs/adding-custom-environment-variables/#what-other-env-files-can-be-used).
+{% /callout %}
+
 ```txt#.env
 FOO=hello
 BAR=world

docs/test/runtime-behavior.md

@@ -12,6 +12,8 @@ test("NODE_ENV is set to test", () => {
 });
 ```
 
+When `NODE_ENV` is set to `"test"`, Bun will not load `.env.local` files. This ensures consistent test environments across different executions by preventing local overrides during testing. Instead, use `.env.test` for test-specific environment variables, which should be committed to your repository for consistency across all developers and CI environments.
+
 #### `$TZ` environment variable
 
 By default, all `bun test` runs use UTC (`Etc/UTC`) as the time zone unless overridden by the `TZ` environment variable. This ensures consistent date and time behavior across different development environments.