Parse worker-saturation if a string

[gjoseph92]

When set via an env var, it may not end up in the config as a float.

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

[github-actions]

Unit Test Results

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

       15 files  ±  0         15 suites  ±0   5h 58m 5s ⏱️ - 4m 29s
  3 114 tests +  2    3 026 ✔️  -   1    85 💤 ±0  3 ❌ +3 
23 050 runs  +16  22 140 ✔️ +12  906 💤 ±0  4 ❌ +4 

For more details on these failures, see this check.

Results for commit c9c2d15. ± Comparison against base commit 17c70e9.

[jrbourbeau]

Seems reasonable -- thanks @gjoseph92