fix(react-router-serve): ensure node_env set before react loaded

[jrestall]

Fixes #12078.

Error only reproducible with React v19 but even with React v18 the dev react and prod react-dom modules are loaded, it just doesn't hard error.

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[brophdawg11]

Thank you for this PR! I'm working on a similar issue with prerendering in React 19 so I cherry-picked these over to https://github.com/remix-run/react-router/pull/12505 and will get the fix in via that PR.