Revert "Makefile: Stop installing binaries into source dir"

This reverts commit eb6fd605.

As described in gitlab#331758 (closed), GitLab's source installs still depend on the Gitaly binaries being installed in root of Gitaly's repository. In eb6fd605, Gitaly's Makefile was updated only place the binaries in the _build/bin directory which is used by other downstream projects.

I've opened gitlab!62456 (merged) and gitlab!62457 (merged) to update the docs to also use the new location of the binaries. Existing installations would still be using the old path as far as I can tell, so we should keep producing binaries in the repo root until we've somehow made sure the existing installations have also been updated to use the new path.

Fixes gitlab#331758 (closed)