#30128 ci(nightly): share compiler-cache settings between warmer, PR Release and nightly Release

The nightly warmer and PR Release build used different Xcode compiler-cache settings from the nightly Release build, while sharing cache keys. This follow-up routes both through the same allowlisted fleet-compilation-cache.sh settings so restored entries use compatible invocation settings.

This addresses the independent review follow-up on #30061 at beb297ffb5f5a77c7c7f539476c3c9f85df42c84 and closes #30106. The regression test checks both workflow steps consume the shared settings helper.

Validation:

• python3 tests/test_ci_fleet_compilation_cache.py (7 tests)
• python3 scripts/cmux-next/prepush-gate.py

Agent session: agent session
🤖 Generated with Claude Code

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Summary by cubic

Aligns the nightly warmer and PR Release Xcode builds with the nightly Release build's compiler-cache settings, so restored cache entries use compatible invocation settings.

• Extracts the allowlisted fleet CAS settings into a shared scripts/ci/fleet-compilation-cache.sh helper, replacing the inline logic in compile-app-host-test-product.sh.
• Wires the warmer and PR Release builds through the same helper; an absent or unhealthy managed CAS keeps each build local.
• Adds the helper to product and incremental build identity tracking so settings changes force a clean build.
• Adds regression tests covering the helper's allowlist behavior and workflow wiring.