[Testing] Use the SDK name for the back-deployment concurrency library path

Author: DougGregor

test/lit.cfg

@@ -1806,7 +1806,7 @@ concurrency_back_deploy_path = ''
 if run_vendor == 'apple':
     if 'back_deploy_concurrency' in lit_config.params:
         config.available_features.add('back_deploy_concurrency')
-        concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', run_os)
+        concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', xcrun_sdk_name)
 
 def os_stdlib_paths():
     if run_vendor == 'apple':