Cleared m_scriptFilename between scene files. (#2278)

This fixes a bug where a Python script file would be run on a scene file
that didn't have a script file set.
Added a general case version to SceneBuilderWorker.cpp, to make it easy
to mark all scene files as dirty.
Automated tests for this will come in a separate pull request.

Signed-off-by: stankowi <4838196+AMZN-stankowi@users.noreply.github.com>

Code/Tools/SceneAPI/SceneData/Behaviors/ScriptProcessorRuleBehavior.cpp

@@ -305,6 +305,11 @@ namespace AZ::SceneAPI::Behaviors
     {
         using namespace AzToolsFramework;
 
+        // This behavior persists on the same AssetBuilder. Clear the script file name so that if
+        // this builder processes a scene file with a script file name, and then later processes
+        // a scene without a script file name, it won't run the old script on the new scene.
+        m_scriptFilename.clear();
+
         if (action != ManifestAction::Update)
         {
             return Events::ProcessingResult::Ignored;