azure: Explicitly set allowBlobPublicAccess to false

[nturinski]

Fixes microsoft/vscode-azurefunctions#4348

According to the SDK, this was always defaulted as false anyway

 /** Allow or disallow public access to all blobs or containers in the storage account. The default interpretation is false for this property. */
    allowBlobPublicAccess?: boolean;

But the new policy explicitly checks for it, I think. So we might as well set it just to be safe.