fix integration tests

zokrates_cli/tests/code/n_choose_k.zok

@@ -2,8 +2,8 @@ import "utils/casts/u32_to_field" as to_field
 
 // Binomial Coeffizient, n!/(k!*(n-k)!).
 def fac(field x) -> field:
-    field f = 1
-    field counter = 0
+    field mut f = 1
+    field mut counter = 0
     for u32 i in 1..100 do
         f = if counter == x then f else f * to_field(i) fi
         counter = if counter == x then counter else counter + 1 fi