Fix scheduled Iceberg performance test checkout

[bvolpato]

Fixes #38694

Scheduled Java IO performance runs build their matrix from master, then check out latest release before running each Gradle task. IcebergPerformanceTest was added after v2.75.0, so scheduled runs consistently fail because that release does not contain :it:IcebergPerformanceTest.

Keep Iceberg performance job on initial master checkout. Existing scheduled performance jobs continue testing latest release, and manually triggered runs remain unchanged.

Validation:

• ./gradlew --no-daemon :beam-test-gha:preCommit
• git diff --check
• Verified IcebergPerformanceTest exists on master and is absent from v2.75.0

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