fix: scope issue when using when with a function call subject

examples/acceptance_tests/036/validators/spend.ak

@@ -1,6 +1,6 @@
 use aiken/dict
 use aiken/list
-use aiken/transaction.{Output, ScriptContext}
+use aiken/transaction.{Output, OutputReference, ScriptContext}
 use aiken/transaction/value.{PolicyId}
 
 const my_policy_id: PolicyId = #"0000000000"
@@ -16,3 +16,19 @@ pub fn spend(_datum: Data, _redeemer: Data, ctx: ScriptContext) -> Bool {
   ctx.transaction.outputs
   |> list.any(has_policy_id(_, my_policy_id))
 }
+
+fn mint(
+  output_reference: OutputReference,
+  _redeemer: Void,
+  ctx: ScriptContext,
+) -> Bool {
+  when
+    list.find(
+      ctx.transaction.inputs,
+      fn(input) { input.output_reference == output_reference },
+    )
+  is {
+    Some(_) -> True
+    None -> False
+  }
+}