Decrease Hamiltonian system test

Trying to solve it for N=8 takes more than half an hour.

.github/workflows/system_test.yml

@@ -43,7 +43,7 @@ jobs:
         include:
           - name: 'Hamiltonian (ZDD)'
             exec: adiar_hamiltonian_zdd
-            N: '1 2 3 4 5 6 7 8'
+            N: '1 2 3 4 5 6 7'
 
           - name: 'Queens (BDD)'
             exec: adiar_queens_bdd