Update FNH worker config to only start worker in placeholder mode.

[kshyju]

Fixes #2501

Changes in this PR leverage the modifications introduced in Azure/azure-functions-host#10231 to disable a worker profile. In this PR, I updated the dotnet isolated worker.config to ensure it starts only in placeholder mode by checking specific environment variable values. If these values are absent, a fallback profile is selected, which disables the worker. This ensures that the dotnet isolated worker does not start after specialization is complete.

Environment variables to be present for the worker to be included:

Windows

1. WEBSITE_PLACEHOLDER_MODE = 1

Linux

1. WEBSITE_PLACEHOLDER_MODE = 1
2. WEBSITE_CONTAINER_READY = 0

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