Makefile: Fix conflicting module graphs for external build tools

When building our build tools, we set up a separate "_build/go.mod" file. The intention is that build tools' dependencies shouldn't be added to the "go.mod" of the actual Gitaly project.

While this works most of the times, recently there had been some issues caused by this where we observe conflicting module graphs when building golangci-lint. The root cause seems to be that its module graph conflicts with the module graph of another build tool and thus fails to build, seemingly because some modules have moved somewhere else.

Let's fix this problem by using per-tool "go.mod" files.