Clean up Makefile

In Gitaly we use a Makefile template and generator to get the benefits of a dynamic Makefile without having to use (IMO hard to understand) dynamic features of Make. The top-level Makefile only exists to render the "dynamic" Makefile (_build/Makefile) and dispatch tasks to that Makefile.

With this MR I am trying to make it a little clearer that we should not use the top-level Makefile to write our Make tasks. The top-level should only dispatch down.

I am also removing some test environment variable complexity that we don't use / care about anymore.