Fix locking in ReflectionComposablePart.ImportsCache (#103660)

The second check needs to use the value from the field (to see updates
made by other threads), not the local variable.

Fixes: #103650

src/libraries/System.ComponentModel.Composition/src/System/ComponentModel/Composition/ReflectionModel/ReflectionComposablePart.cs

@@ -87,6 +87,7 @@ private Dictionary<ImportDefinition, ImportingItem> ImportsCache
                 {
                     lock (_lock)
                     {
+                        value = _importsCache;
                         if (value == null)
                         {
                             value = new Dictionary<ImportDefinition, ImportingItem>();