cryptol: Make Z3 available for REPL users

The Cryptol REPL has a hard dependency on Z3, but the rest of the
library uses SBV to support multiple solvers. Ensure that Z3 is
available for `pkgs.cryptol`, which is likely to be installed via
nix-env for REPL usage, but do not change pkgs.haskellPackages.cryptol,
which is likely to be used as a dependency (in Nix expressions).