Input Blob Binding name fails with an activity trigger

🐛 **Describe the bug**
Input Blob Binding name fails with an activity trigger

🤔 **Expected behavior**
activity function takes a string argument (the name of a blob) and uses that value to bind to blob input

☕ **Steps to reproduce**

try to bind a blob name to the value passed in an activity trigger

{
"scriptFile": "__init__.py",
"bindings": [
{
"name": "name",
"type": "activityTrigger",
"direction": "in"
},
{
"name": "inputb",
"direction": "in",
"type": "blob",
"path": "container/{name}.json",
"connection": "AzureWebJobsStorage",
"dataType": "string"
}
]
}

Should work like explained in this link but fails in Pyhon
https://github.com/Azure/azure-functions-durable-extension/pull/172

Start by reproducing the activity trigger and blob binding configuration shown in the issue, using __init__.py as the script entry point. Compare the Python behavior with the referenced durable-extension PR and trace how the activity argument is passed into the blob path. Done means a blob named from the activity argument is successfully bound as input.

Written by the indexing model from the issue text.