Resolve setup-env from the Metro project root

[OskarEichler]

Summary:

@react-native/metro-config currently resolves react-native/setup-env relative to its own package location. In strict or multi-version installs, that can select a different React Native copy—or fail—even though the configured projectRoot contains the correct export.

Resolve the bootstrap module with require.resolve(..., {paths: [projectRoot]}), matching community-cli-plugin's existing project-scoped behavior. This does not change normal single-version layouts; it makes the existing projectRoot contract deterministic across package-manager layouts.

Changelog:

[GENERAL] [FIXED] - Resolve the React Native setup module from the configured Metro project root.

Test Plan:

• Built metro-config and verified the generated configuration resolves the same react-native/setup-env as require.resolve(..., {paths: [projectRoot]}) for a separate React Native project.
• A synthetic strict project fixture reproduced the baseline selecting the repository React Native; the patch selected <projectRoot>/node_modules/react-native/setup-env.js.
• yarn build metro-config
• fresh full Flow check: 0 errors
• targeted ESLint and Prettier checks
• git diff --check

metro-config currently has no Jest tests. Its source wrapper loads index.flow.js, which the root Jest Node transform leaves as ESM, so a direct package test fails before execution; this change therefore uses built-output resolution verification rather than adding a bespoke compile/eval-only harness for the one-line fix.