[Linux Consumption] Don't block assignment call on sidecar specialization

Make this call non-blocking
https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script.WebHost/Controllers/InstanceController.cs#L54

For apps that take longer to specialize sidecar, this call can lead to request timeout on worker assignment instead of function invocation. Only impacting small number of apps.