Bump Z3 version (#3551)

Related to
runtimeverification/evm-semantics#1499, I'm
going through the K tools and semantics checking whether upgrading the
supported Z3 version we require is going to break anything.

---------

Co-authored-by: Jost Berthold <[email protected]>
Co-authored-by: Georgy Lukyanov <[email protected]>

Author: 3 people

flake.lock

@@ -4,7 +4,7 @@
     haskell-nix.url = "github:input-output-hk/haskell.nix";
     nixpkgs.follows = "haskell-nix/nixpkgs-unstable";
     z3-src = {
-      url = "github:Z3Prover/z3/z3-4.8.15";
+      url = "github:Z3Prover/z3/z3-4.12.1";
       flake = false;
     };
     flake-compat = {