Fix DISABLE_PUMA_WORKER_KILLER env var check

What does this MR do?

Equivalent fix to gitlab!88928 (merged)

The check for DISABLE_PUMA_WORKER_KILLER was not correct, since it hosts a string value. This can lead to the PWK not running whenever the env var carries some non-nil value, including 1 or true, 0 or false.

Related issues

gitlab#365954 (closed)

Test plan

• Set DISABLE_PUMA_WORKER_KILLER to true or 1
• The PWK should now be running (look for PumaWorkerKiller logs in puma.stdout)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• [-] Documentation created/updated; nothing to document here.
• [-] Tests added; this code is executed during Puma start-up, so I don't think we can test this here.
• [-] Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened. I believe this needs a fix in CNG, not the chart, which is here: gitlab-org/build/CNG!1042 (merged)