fetchgit: import impure GIT_PROXY_COMMAND and SOCKS_SERVER, fixes #8605

These environment variables allow using fetchgit with git:// URLs using
the SOCKS proxy technique described in 'Using Git with a SOCKS proxy':

  http://www.patthoyts.tk/blog/using-git-with-socks-proxy.html

Briefly, GIT_PROXY_COMMAND is set to a script which invokes connect[1],
which reads SOCKS_PROXY, which might be pointing to a local instance of
'ssh -D'.

[1] pkgs/tools/networking/connect