writing VCD for combinatorial circuits works!

crates/fayalite/src/cli.rs

@@ -653,6 +653,7 @@ impl FormalArgs {
                 self.sby.display()
             )))
         };
+        if do_cache {
             fs::write(
                 cache_file,
                 serde_json::to_string_pretty(&FormalCache {
@@ -666,6 +667,7 @@ impl FormalArgs {
                 })
                 .expect("serialization shouldn't ever fail"),
             )?;
+        }
         result
     }
 }