The go-modules approach is a bit broken: https://github.com/golang/go/issues/29228
To resolve that, we now compute that version using `git describe` and
set it using `ldflags`.
While this is not as nice/builtin as the other approach, it at least
works all the time and across go versions.
Furthermore, I did refactor the Makefile in preparation for an AUR
package that builds from source and for building release artifacts in CI.
