No support for authenticated proxies

### Describe the bug

Desktop fails a clone with a HTTP/407

### Version & OS
macOS 10.15.2
Tried GA and 2.5.3-beta1

### Steps to reproduce the behavior

1. Clone repo

### Expected behavior

Repo is cloned

### Actual behavior

Error noting the 407 because it couldn't handle the authentication.

In the log I could see
2020-06-16T19:15:10.777Z - info: [ui] proxy url not resolved, https_proxy already set
Unclear if this is from gitconfig or envvar. Tried to make sure they were in sync, but always get the 407 even w user:pass@proxyurl in the envvar.

### Additional context

Be nice to offer a fallback to use the git client as it handles the 407 with existing proxy statements in gitconfig including prompting for password via helper if needed. Rust's cargo has a flag (git-fetch-with-cli = true) to do this similar operation as the mix of configs, envvars, and system prefs can sometimes be challenging to figure out.