What problem does this feature solve?

Description
Currently, loadEnv() merges environment variables from .env,.env[.local] .env[.mode], and .env[.mode[.local]] files into process.env. While useful for build processes, this causes environment pollution when using custom CLI tools outside rsbuild's build/dev flows.

The current loadEnv() + cleanup() pattern has limitations when environment variables already exist before loading:

1. cleanup simply deletes injected variables rather than restoring original values
2. Pre-existing values get permanently erased when using this pattern

Proposal
Add a dryRun boolean option to loadEnv(). When enabled:

1. Files are still loaded/merged using existing priority rules
2. Parsed values are returned in the parsed field
3. No modifications are made to process.env

(Note: loadEnv() May need a separate processEnv configuration for advanced environment injection control,like:https://dotenvx.com/docs/advanced/parse-process-env)

What does the proposed API look like?

Use Case
Creating custom CLI tools that need env values without side effects:

# .env
HELLO=WORLD

import { loadEnv } from '@rsbuild/core';

// In custom CLI script
const { parsed } = loadEnv({
  dryRun: true, // Prevent env injection
  mode: 'production'
});

console.log(parsed.HELLO); //   WORLD   Value from .env files
console.log(process.env.HELLO); // undefined (no pollution)

Behavior Comparison

Implementation Notes

1. Should maintain full compatibility with existing API
2. Should preserve existing environment variable merging logic
3. Documentation update needed for new option