Unable to set default values

```
local getExt(varName, default) =
if std.extVar(varName) != null then std.extVar(varName) else default;

{
machine: {
gpu: 0,
cpu: 100,
ram: 100,
},
job: {
APIVersion: "V1beta1",
Spec: {
Deal: {
Concurrency: 1,
},
Docker: {
Entrypoint: [
"/usr/games/cowsay",
getExt("Message", "Pass me an input called Message, like darts run cowsay:v1.0.0 -i Message=moo")
],
Image: "ghcr.io/darts2024/cowsay:v1.0.0",
},
Engine: "Docker",
Network: {
Type: "None",
},
PublisherSpec: {
Type: "local",
},
Resources: {
GPU: "",
},
Timeout: 500,
Verifier: "Noop",
},
},
}

```
###Error

```
jsonnet default.dart
RUNTIME ERROR: Undefined external variable: Message
default.dart:2:6-25 function
default.dart:19:11-108 thunk from >
Array element 1
Field "Entrypoint"
Field "Docker"
Field "Spec"
Field "job"
During manifestation
```

Version:

```
Jsonnet commandline interpreter (Go implementation) v0.20.0
```