This is because you use Shared access mode.
This enables multiple users to use the cluster simultaneously.
However, there are features that do not work on these Shared access mode clusters:
https://docs.databricks.com/en/compute/access-mode-limitations.html#shared-access-mode-limitations-o...
The limitation about backticks in env vars is not mentioned but the error message is clear: it is not supported.
To overcome this limitation, use Single user as access mode.