Backport of 'Ignore CI variable in roulette experiment default specs'

What does this MR do and why?

Backport of !262336 (merged) to 19-5-stable-ee. This is the same fix as the 19-4-stable-ee backport in !262377 (merged).

The project-level CI variable ROULETTE_HIDE_REVIEWER_COLUMN_PERCENT is set to 100 and leaks into spec/tooling/danger/roulette_experiment_spec.rb. The two examples that check the default 50% split fail on every 19-5-stable-ee pipeline that runs this spec. This change stubs the variable to nil for the .hide_reviewer_column? examples, so the default applies. It only changes the spec, and the commit cherry-picked cleanly from 19491c5ea1670d22fad3b988db3a64270f8ce427.

Context and local verification

19-5-stable-ee still has the pre-fix spec, so every MR that targets it fails rspec unit. That includes !262488 (merged), !262498 (merged), and !262206. These failures add to the blocked-pipeline count on the CI health incident.

Ran on this branch:

• Original stable spec, ROULETTE_HIDE_REVIEWER_COLUMN_PERCENT=100: 11 examples, 2 failures (:14, :21)
• Backported spec, ROULETTE_HIDE_REVIEWER_COLUMN_PERCENT=100: 11 examples, 0 failures
• Backported spec, variable unset: 11 examples, 0 failures

References

• gitlab-org/quality/analytics/ci-health-incidents#1376 (closed)
• !262336 (merged)
• !262377 (merged)

MR acceptance checklist

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

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.