fix(load-tests): honor INVOKE_HOST in supreme_http_test

[vrv3814]

Summary

• run_http_self_managed_test.sh from http load-testing docs exports INVOKE_HOST=invocation.<gateway> and forwards it via -e, but supreme_http_test.js never read it, so all requests sent Host: <gateway> and were 404'd by the
  NVCF gateway's host-based routing.
• Set params.headers.Host = __ENV.INVOKE_HOST when the variable is provided. k6 promotes a Host header to req.Host, so the request now lands on the invocation.* vhost.
• Gated on INVOKE_HOST being set so other consumers of this script are unaffected.

Checklist

• I am familiar with the Contributing Guidelines.
• I have signed off my commits for Developer Certificate of Origin (DCO) compliance.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.