Fix nexus cancellation type test flake

[dandavison]

Fix races in nexus cancellation tests.

I have not repro'd the failures on my MacOS laptop; this PR was tested by modifing CI to run cancellation types tests 10 times.

• Move a future out of the global struct into a workflow; exposed via an update
• Stop making assertions about CANCEL_REQUEST_{COMPLETED,FAILED} under TRY_CANCEL since in that case the timing of these events (which are triggered by cancel handler returning) races with the timing of the events triggered by the handler workflow exiting as canceled.
• Use workflow.now() instead of recording clock time in the workflow.