This is a second fix for the GRPC bug related to using unix sockets
alongside unix sockets. Previously we fixed incoming clients. This
change fixes the connection between Gitaly and Gitaly-Ruby, which
always relies on unix sockets, and which currently fail when http
proxies are setup.

A (too) long history of this change:

* Unix sockets have never been officially supported by gRPC
* Originally unix sockets didn't work at all, and you had to use a
  dialer
* Attempted to upgrade from gRPC 1.9 to 1.16 and found that our dialer
  broke connectivity:
  !972
* Isolated the problem to this commit:
  https://github.com/grpc/grpc-go/commit/90dca43332f6cc944c37e16f32a82c41639e7705
* Fixed the problem by switching to the default dialer for unix: URLs
* (note that it's still not officially supported, but without
  integration tests with proxy configurations, it seems to work)
* In #1447, discovered that the default dialer...