Fix Nanny shutdown edge case

[gjoseph92]

Introduced in #6146, which was putting a close message instead of stop:

distributed/distributed/nanny.py

Lines 855 to 862 in b8b45c6

	try:
	msg = child_stop_q.get()
	except (TypeError, OSError, EOFError):
	logger.error("Worker process died unexpectedly")
	msg = {"op": "stop"}
	finally:
	child_stop_q.close()
	assert msg["op"] == "stop", msg

This error was triggered in #6320, but clearly doesn't happen in CI normally. I have not tried to add a test for it. It was already untested when added.

cc @mrocklin @fjetter

• Tests added / passed
• Passes pre-commit run --all-files

[github-actions]

Unit Test Results

       15 files  ±0         15 suites  ±0   6h 50m 34s ⏱️ - 13m 14s
  2 792 tests ±0    2 709 ✔️  - 5    79 💤 +1  3 ❌ +3  1 🔥 +1 
20 707 runs  +1  19 795 ✔️  - 1  908 💤  - 2  3 ❌ +3  1 🔥 +1 

For more details on these failures and errors, see this check.

Results for commit 5092403. ± Comparison against base commit 63cdddd.

[mrocklin]

My apologies. Thanks