Reenable Git2Go integration

With commit a6091637 (Merge branch 'pks-revert-git2go' into 'master', 2020-07-31), our new Git2Go dependency was reverted again due to several issues:

- Some distributions didn't have a recent enough version of
  CMake available. As a result, building libgit2 failed. This was
  fixed by bumping the build images' CMake version.

- Build tags didn't work as expected and thus we ended up not using
  them at all. This was fixed by passing build flags via the
  Makefile. As this causes e.g. `go test ./...` to fail with a
  linking error due to build tags not being specified and the
  PKG_CONFIG_PATH not being set up as required. To work around this
  issue and not break developer workflows, gitaly-git2go will now
  only be built if build tags required for git2go are set.

- Invalidation of libgit2.a didn't work as required. E.g. if the
  Makefile changes, it wasn't rebuilt and it was thus impossible to
  provide fixes for any broken setups if libgit2.a was built
  already. This was fixed by adding a build dependency on the
  Makefile.

All in all, this should fix all currently known problems with building libgit2.