Fix HTTP proxies not working in Gitaly hooks

Previously HTTP proxy environment variables (e.g. HTTP_PROXY, HTTPS_PROXY, no_proxy) were passed through to Git processes, which then executed hooks (e.g. pre-receive). That way access to the GitLab internal API could pass through a proxy if necessary.

When the responsibility for executing these Git hooks was passed to Gitaly, this stopped working. We need to allow these same environment variables for proxies to work.

Closes #2913 (closed)