Fix Unix socket address handling following gRPC upgrade

In 29f8bbbc, we upgraded our gRPC library version. The upgrade introduced some differences in how the Unix socket addresses are handled. We've previously prefixed unix addresses always with 'unix://'. In the new version of gRPC, this does not work for relative paths as the first section following the slashes are interpreted as an authority. Instead, we should either uses 'unix:relative/path', 'unix:/absolute/path' or 'unix:///absolute/path'. As it is, Gitaly configurations that are using relative paths for the internal socket directory do not work as it results in address 'unix://relative/path'. This MR changes the two non-test locations I found by grepping for 'unix://' to prefix the the socket only by 'unix:' so both relative and absolute paths work.

Closes #3664 (closed)