On Mac, building `gargantext` requires linking against the
Accelerate framework (completely unrelated to the `accelerate` Haskell
package!). This patch provides an extra `pkgs.nix` directive that will
kick in only when building on Darwin.

Remove joins, use optionalRestrict instead.

Also, remove cabal.project because stack.yaml and package.yaml cover
dependencies already.

Based on the suggestions in
https://kodimensional.dev/space-leak