release-25.4: 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 #164315 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: This PR qualifies as compliant based on the backport policy for CockroachDB. The PR changes only a test file (pkg/sql/schema_changer_test.go), as indicated by the '_test.go' suffix, which means it falls under the category of non-production code changes. Therefore, the changes are exempt from the standard backport policy requirements that typically necessitate fixing a critical bug or being gated behind a feature flag. Moreover, the PR body explicitly includes a 'Release justification: test only change,' further supporting its exemption from the standard requirements given it only affects the test code. These factors confirm that the PR is indeed compliant with the backport policy, as it neither alters production code nor requires additional gating.

✅ 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