[Python CSM] Fix interop client and server flag (#36827)

We changed the way flag was passed in this PR: https://github.com/grpc/psm-interop/pull/94
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Closes #36827

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36827 from XuanWang-Amos:csm_fix_interop a2c2c47f30
PiperOrigin-RevId: 640908453

src/python/grpcio_tests/tests_py3_only/interop/xds_interop_client.py

@@ -670,8 +670,8 @@ if __name__ == "__main__":
     parser.add_argument(
         "--enable_csm_observability",
         help="Whether to enable CSM Observability",
-        default=False,
-        action="store_true",
+        default="False",
+        type=bool_arg,
     )
     parser.add_argument(
         "--request_payload_size",

src/python/grpcio_tests/tests_py3_only/interop/xds_interop_server.py

@@ -218,8 +218,8 @@ if __name__ == "__main__":
     parser.add_argument(
         "--enable_csm_observability",
         help="Whether to enable CSM Observability",
-        default=False,
-        action="store_true",
+        default="False",
+        type=bool_arg,
     )
     args = parser.parse_args()
     if args.verbose: