Ability to use a variable with Bitbucket runner 'runs-on' statement

Currently, Bitbucket runner 'runs-on' on statement on the YML file does not allow using variables (workspace, repository and deployment variables) to specify which runner to run, and throws the following error :

"There is an error in your bitbucket-pipelines.yml at [pipelines > custom > initial-testing > 0 > step]. To be precise: This runs-on label: $runner is invalid. Only alphanumeric characters and dots can be used in runs-on labels."

 - step:
name: Initial testing
runs-on:
- $runner 

Where: $runner is a deployment variable.

It would be nice to have the ability to use the variables to specify the runner label.