Webpack loaders that mutate `process.env` at build time not supported

> *This issue was created by an agent analysing CI failures from the [Next.js Deploy Suite](https://github.com/cloudflare/vinext/actions/runs/26282973805) (vinext `main` vs Next.js `v16.2.6`, 2026-05-22).*

## Problem

Next.js allows webpack loaders to set environment variables during the build (e.g. via `process.env.X = ...`), which then become available in user code. vinext (Vite/Rolldown-based) does not run webpack loaders, so these env vars are absent and the test app renders the default error page.

```
Webpack loader sets process.env.X at build time; user code reads undefined
```

## Estimated Impact

~1 test failures across the deploy suite.

## Affected Test Suites

- `test/e2e/app-dir/webpack-loader-set-environment-variable/webpack-loader-set-environment-variable.test.ts`

## Recommendation

1. **Reproduce first in vinext's own test suite.** Add a fixture with a webpack loader that sets `process.env.MY_VAR` and a page that prints `process.env.MY_VAR`. Confirm the page does not see the value.

2. **Decide on a vinext equivalent.** Either implement a Vite plugin that runs at build start and can mutate the `define` map, or document that webpack loaders are not portable. Update fixture / triage decision accordingly.