Freebuff does not use HTTP_PROXY env variable

feat: Add HTTP proxy support for corporate/enterprise users in (https://github.com/CodebuffAI/codebuff/commit/7bd2afa9ef996181d47074499fb4b059772c85a1) only fixes downloading codebuff or freebuff executable using http proxy. But freebuff does not use HTTP_PROXY. So freebuff is not usable behind corporate/enterprise proxy servers. 