[test] Fix proxy-request-with-middleware deploy test

[eps1lon]

The fixture's API route proxies the incoming request to http://<host>/api/post. On Vercel, that plain-HTTP hop hits the edge's http→https upgrade and the handler pipes the resulting 308 back to the test client, which then has to replay the POST body across the redirect. Node.js' fetch implementation cannot replay bodies across 307/308 redirects and fails the request with UND_ERR_REQ_CONTENT_LENGTH_MISMATCH, which broke this test in deploy mode after the test harness switched from node-fetch to global fetch in #98195.

This change proxies over https when VERCEL_URL is set so no client-visible redirect occurs, while local dev and start modes are unaffected since they keep using plain HTTP.

Fixes

TypeError: fetch failed

      29 |         'x-custom-header': 'some value',
      30 |       }
    > 31 |       const res = await fetchViaHTTP(next.url, `api`, '', {
         |                   ^
      32 |         method: method.toUpperCase(),
      33 |         headers,
      34 |         body: method.toLowerCase() !== 'get' ? body : undefined,

      at Object.<anonymous> (e2e/proxy-request-with-middleware/test/index.test.ts:31:19)

    Cause:
    RequestContentLengthMismatchError: Request body length does not match content-length header

-- https://github.com/vercel/next.js/actions/runs/34543344283/job/103105861941#step:35:555

[github-actions]

Tests Passed

Commit: 6011e03

[eps1lon]

change detection is currently crude so this was required to get confirmation from a real run: https://github.com/vercel/next.js/actions/runs/34592487556/job/103242566944?pr=98553#step:35:102