listkeys function should not work as output using variable assignment

^{Originally posted by **vlahane** December 14, 2022}
Using below bicep code (example) able to pass keys as output using variable & deploy, I think we should give some error
as we are not allowing outputing keys like `output accountKeys = '${listKeys(storageAccount.id, '2019-04-01').keys[0].value}'
` which gives expected error: `Outputs should not contain secrets. Found possible secret: function 'listKeys'` but below example works fine.

example:
```

param storgeAccountName string
param location string
resource storageAccount 'Microsoft.Storage/storageAccounts@2022-09-01' = {
name: storgeAccountName
location: location
kind: 'StorageV2'
sku: {
name: 'Standard_LRS'
}
properties: {
accessTier: 'Hot'
}
}

var keys = '${listKeys(storageAccount.id, '2019-04-01').keys[0].value}'
output accountKeys string = keys
```
` tl;dr:` If above (example) output keys using variable is not recommanded then ideally bicep should not allow assigning listKeys to variable as well or does listkeys output supported via variable assignment if so then we still exporting secrets using outputs?

`Bicep version : 0.13.1`