release-24.3: sql: deflake TestIndexBackfillAfterGC

[fqazi]

Backport 1/1 commits from #164297 on behalf of @fqazi.

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Skip TestIndexBackfillAfterGC under race, since it can be timing sensitive.

Fixes: #161759

Release note: None

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Release justification: test only change

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes OR fixes for serious issues. Non-production includes test-only changes, build system changes, etc. Serious issues are defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to. Reference the approved ENGREQ ticket in the PR body (e.g., "Fixes ENGREQ-123").

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

✅ PR #164313 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request is a test-only change which is exempt from the standard backport policy requirements. The modified file, pkg/sql/schema_changer_test.go, clearly indicates that the change is within a test function TestIndexBackfillAfterGC. The added line uses a function to skip the test under certain conditions, labeled as 'flaky under race'. This falls within the policy exception for non-production changes. Moreover, the PR body explicitly mentions 'Release justification: test only change', further supporting the exemption based on test-only modifications.

✅ ENGREQ Check Skipped: Test-only backport.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is