Fix missing path separator in prepare_resources-mac.py (#599)

Use os.path.join() instead of string concatenation to properly join
TARGET_BUILD_DIR and UNLOCALIZED_RESOURCES_FOLDER_PATH. Without this fix,
resources were copied to an invalid path (e.g. '/Users/name/ApplicationsApp.app'
instead of '/Users/name/Applications/App.app'), causing the app to crash on
startup due to missing UI resources.

Author: adamnsch

NeuralAmpModeler/scripts/prepare_resources-mac.py

@@ -31,9 +31,9 @@ def main():
             + "/Resources"
         )
     else:
-        dst = (
-            os.environ["TARGET_BUILD_DIR"]
-            + os.environ["UNLOCALIZED_RESOURCES_FOLDER_PATH"]
+        dst = os.path.join(
+            os.environ["TARGET_BUILD_DIR"],
+            os.environ["UNLOCALIZED_RESOURCES_FOLDER_PATH"]
         )
 
     if os.path.exists(dst) == False: