Update z3 for vtock

Author: nilehmann

.github/workflows/ci.yml

@@ -44,15 +44,11 @@ jobs:
       - name: Install fixpoint
         uses: ./.github/actions/install-fixpoint
 
+      # Older versions hang
       - name: Install Z3
         uses: cda-tum/[email protected]
         with:
-          version: 4.12.1
-
-      - name: Install CVC5
-        uses: ./.github/actions/install-cvc5
-        with:
-          version: 1.2.1
+          version: 4.15.3
 
       - name: Rust Cache
         uses: Swatinem/[email protected]