Assign locked resource names with extra keyword

I would like to run the following block before a `states`-block:

```
options {
lock(extra:
[
[label:'test-instance', variable: 'TEST_INSTANCE', quantity: 1, resource: null],
[label:'remote-dongle', variable: 'HOSTNAME', quantity: 1, resource: null],
[label:'local-dongle', variable: 'LOCALNAME', quantity: 1, resource: null]
],
inversePrecedence: false,
variable: 'LOCKED_RESOURCES'
)
}
```
I get a comma-separated list of the names of the 3 locked resources in `${LOCKED_RESOURCES}`. However, the order of the resource names is random and varies every time I run the pipeline. That is why I need to assign `variable` on a per-locked-resource base, which however currently does not work (the variables `${TEST_INSTANCE}`, `${HOSTNAME}`, `${LOCALNAME}` are simply `null`).