Function hardcodes the runtime to nodejs18.x and fails to build with layers with different runtimes

[Amousy]

Hello,

I'm reposting the issue here as it hasn't been fixed yet.
sst/sst#3887

---

I use the datadog-cdk-constructs-v2 livrairie to automatically add the Datadog layer and extension to all the lambdas in my Stack.

` app.setDefaultFunctionProps({
runtime: "nodejs20.x",
});

app.node.children.forEach((stack) => {
const datadog = new DatadogLambda(stack, "datadog", {
nodeLayerVersion: 115,
extensionLayerVersion: 65,
});

      datadog.addLambdaFunctions(stack.getAllFunctions());
    }
  });

`

We do have the nodejs20.x for Lambdas but wrong nodejs version in layer cause SST Function hardcodes the runtime to nodejs18.x

[jayair]

Happy to look at a PR for this.