Transient "Did not find functions with language [python]" errors

Occasional `"Did not find functions with language [python]"` error (does not impact availability)

1. App starts in placeholder mode
2. Customer's contents begin to get downloaded and extracted (not sure why?)
3. 1 function is indexed (I think the warmup function)
4. Job Host started
5. 2 function and 1 proxy are indexed (not sure what these are)
6. Specialization is triggered (going out of placeholder mode)
7. Utility.VerifyFunctionsMatchSpecifiedLanguage is called here from GetFunctionDescriptorsAsync in ScriptHost. At this point, the app could no longer be in placeholder mode but still have placeholder mode descriptors.

This seems to be caused by a race condition with specialization.

There was a fix here to address a similar issue: [Use CancellationToken during ScriptHost Initialization](https://github.com/Azure/azure-functions-host/pull/6265) but I think there may still be an opportunity for a race condition, because we check if we're in placeholder mode via environment variable.

cc: @pragnagopa and @divyagandhisethi