Can SnapshottingCollection.GetSnapshot() call CreateSnapshot() multiple times?

This code checks that `snapshot == null` before taking a lock, but never checks it again:

https://github.com/dotnet/sdk/blob/88ec7422ea36b15860ea29ec04cc8f8a2c60974f/src/Cli/dotnet/Telemetry/PersistenceChannel/SnapshottingCollection.cs#L79-L92

I think this sequence of events can result in multiple calls to `CreateSnapshot()`:

1. Thread 1 calls `GetSnapshot()` and see `localSnapshot = snapshot` && `localSnapshot == null`
2. Thread 2 calls `GetSnapshot()` and see `localSnapshot = snapshot` && `localSnapshot == null`
3. Thread 1 enters the lock block, and runs CreateSnapshot, and updates `snapshot`
4. Thread 2 enters the lock block, runs CreateSnapshot again, and updates `snapshot` again.