Add FIPS environment variable

What does this MR do and why?

Describe in detail what your merge request does and why.

Adds a FIPS environment variable that can be picked up by GitLab QA scenarios.

This will allow FIPS smoke test to run https://gitlab.com/gitlab-org/distribution/reference-architecture-tester/-/merge_requests/58

The env variable is picked up by https://gitlab.com/gitlab-org/gitlab/-/blob/master/qa/qa/support/fips.rb and used to determine in appropriate tests whether to use the API (non-FIPS) or Browser to fabricate https://gitlab.com/gitlab-org/gitlab/-/blob/c605fc38a4e0e7911b68f1b8b1f8117447155c3e/qa/qa/resource/base.rb#L25

For context - token creation has been removed from FIPS/Fedramp builds (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/97299), which means that API tokens cannot be used

Further context - https://gitlab.com/gitlab-org/gitlab/-/issues/377938

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

FIPS_MODE=1 gdk restart

FIPS=1 CHROME_HEADLESS=false gitlab-qa Test::Instance::Any gitlab/gitlab-ee-qa:latest http://gdk.test:3000/ -- qa/specs/features/browser_ui/2_plan/issue/create_issue_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.