returns() doesn't override returnsArg()

[MartinKolarik]

Describe the bug

Since #2593, calling returns(...) on a stub that was previously configured with returnsArg(0) doesn't update the behavior. Instead, the original returnsArg(0) behavior is preserved.

To Reproduce
Steps to reproduce the behavior:

const stub = sandbox.stub();

stub.returnsArg(0);
console.log(stub(1));

stub.returns(42);
console.log(stub(2)); // should log 42, logs 2

Context (please complete the following information):

• Sinon version : 20.0.0
• Runtime: Node 20
• Output of npx envinfo --browsers --binaries:

N/A

• Other relevant environmental info:
• Other libraries you are using:
• Example URL:

Additional context
Add any other context about the problem here.