Always generate the version through git when possible, and encode that within
the binaries so that `catala --version` does'nt give misleading information.
Previously we used dune's builtin functionality, but that resorts to a hack at
install time which is unpleasant and doesn't work with our use of `opam
install`.
The cost is a re-linking of catala_utils and the binaries upon git commit, which
is hardly noticeable.
