Fix code capture inside pytest-xdist

[crusaderky]

Fix breakage in coiled/benchmarks caused by #8198.

This is due to a regression that causes a crash whenever you run a Client with code capture enabled from a unit test using pytest-xdist (which in turn is based on execnet)

/usr/share/miniconda3/envs/test/lib/python3.9/site-packages/distributed/client.py:3163: in _graph_to_futures
    computations = self._get_computation_code(
[...]
>               module_name = sys.modules[fr.f_back.f_globals["__name__"]].__name__  # type: ignore
E               KeyError: '__channelexec__'

I manually tested this in coiled/benchmarks.
No unit test included as it's impossible to test without execnet and I'm uneager to add it to the environment just for this.

Output of coiled/benchmarks on the Coiled dashboard:

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       21 files  ±  0         21 suites  ±0   11h 27m 31s ⏱️ + 25m 4s
  3 815 tests ±  0    3 694 ✔️  - 8     107 💤 ±0  14 ❌ +8 
36 875 runs  +12  35 059 ✔️ +8  1 799 💤  - 5  17 ❌ +9 

For more details on these failures, see this check.

Results for commit ff69696. ± Comparison against base commit a74f7cf.