Skip nexus cancellation tests for time-skipping environment

The test_cancellation_type tests were hanging indefinitely when run with
the time-skipping environment, causing CI failures. These tests work fine
with the regular environment but have compatibility issues with time-skipping,
similar to other Nexus tests.

Fixes the CI timeout failures seen on PR #981.

Author: dandavison

tests/nexus/test_workflow_caller_cancellation_types.py

@@ -168,6 +168,9 @@ async def test_cancellation_type(
     env: WorkflowEnvironment,
     cancellation_type: Optional[str],
 ):
+    if env.supports_time_skipping:
+        pytest.skip("Nexus tests don't work with time-skipping server")
+    
     client = env.client
 
     async with Worker(