Respect ENV and config variables when initializing Bullet

What does this MR do and why?

It fixes setting up bullet gem in various configuration scenarios. Currently it's not possible to disable bullet without going into codebase. ENV variable and config variable are not respected

Here's proper configuration matrix:

ENV['ENABLE_BULLET']	gitlab.yml:bullet.enabled	Rails.env	Result
nil	nil	false	false
nil	nil	true	true
nil	false	true	false
nil	true	false	true
false	true	true	false
true	false	false	true

Keep in mind that if bullet gem is not loaded it will not be configured regardless of the matrix above.

References

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/538847#note_2484088032

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #541111 (closed)