Gitaly needs to locate auxiliary binaries in various places to pass
them as arguments and call them. Currently, the logic to do so is
spread around the code base where the cfg.BinDir is joined with the
binary name. Soon we'll have to make a distinction between binaries
that are in the BinDir and binaries that are unpacked into a runtime
directory on start up. To support this, this commit centralizes the
binary finding logic to BinaryPath helper added to Gitaly's config.
For now, it simply join BinDir with the binary name as each of the
call sites were doing. In a later commit, it will also help with
locating the unpacked binaries correctly.