Remove leaking reference to workers from gen_cluster

[hendrikmakait]

Extracted from #6336 to merge independently and enable using this for working on garbage collection of Worker instances.

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

[github-actions]

Unit Test Results

       15 files  +       3         15 suites  +3   6h 59m 41s ⏱️ + 1h 35m 21s
  2 774 tests ±       0    2 693 ✔️ +     10    79 💤  -   12  2 ❌ +2 
20 580 runs  +3 962  19 671 ✔️ +3 847  907 💤 +113  2 ❌ +2 

For more details on these failures, see this check.

Results for commit 0e4175e. ± Comparison against base commit 2b04d8b.

[mrocklin]

Thanks @hendrikmakait !