Fix the logic for QA User fabricate or use

What does this MR do and why?

Describe in detail what your merge request does and why.

Fix logic for the fabricate_or_use QA User function.

If we are in FIPS mode we never want to fabricate using API, as we don't use tokens. signup_disabled is existing functionality and should continue to choose the correct fabrication method if FIPS is not enabled.

Screenshots or screen recordings

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How to set up and validate locally

To test variations, to start GDK in FIPS mode:

FIPS_MODE=1 gdk restart

or non-FIPS mode:

FIPS_MODE=0 gdk restart

Run test eg. Signup disabled true and FIPS mode true (should always use Browser UI fabrication)

SIGNUP_DISABLED=true FIPS=1 CHROME_HEADLESS=false bundle exec bin/qa Test::Instance::All http://gdk.test:3000/ ./qa/specs/features/browser_ui/2_plan/issue/create_issue_spec.rb

eg. Signup disabled true and FIPS mode false (should use API fabrication)

SIGNUP_DISABLED=true FIPS=0 CHROME_HEADLESS=false bundle exec bin/qa Test::Instance::All http://gdk.test:3000/ ./qa/specs/features/browser_ui/2_plan/issue/create_issue_spec.rb

MR acceptance checklist

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