Bump z3 to v4.13.4 (#4083)

This PR updates Z3 to its latest version. It was observed that this
version is performing much more efficiently in Kontrol engagement
proofs, which require reasoning about complex non-linear arithmetic.

The improvements in performance are as follows (will be updated as I
collect more information):
- SAT checks that would take ~64 seconds in v4.13.0 now take ~3 seconds.

Author: PetarMax

flake.lock

@@ -5,7 +5,7 @@
     nixpkgs.follows = "rv-utils/nixpkgs";
     stacklock2nix.url = "github:cdepillabout/stacklock2nix";
     z3 = {
-      url = "github:Z3Prover/z3/z3-4.13.0";
+      url = "github:Z3Prover/z3/z3-4.13.4";
       flake = false;
     };
   };