ci: keep a macOS native build cache on main #6390

What is the problem the feature request solves?

Every macOS build in the merge queue compiles the native library from scratch, which takes about 31 minutes on macos-14. pr_build_macos.yml saves its cargo cache only when github.ref == 'refs/heads/main', but the macOS workflow runs only in the merge queue and on pull requests with the run-macos-tests label, never on main. So the entry is never written and every restore misses.

Today the macOS path (31 minute native build plus the 39 minute [expressions] suites, about 70 minutes) is shorter than the Spark SQL path, but once the Spark SQL shards are rebalanced (#6388) it becomes the longest path in a queue run.

Describe the potential solution

Give the macOS build the same push-to-main cache refresh that the Linux build has:

• a build_macos_full POLICY output next to build_macos, the way build_linux_full sits next to build_linux, with build_macos gaining the push tier
• a cache-refresh-only input on pr_build_macos.yml that skips the test matrix on push
• in that mode, build-native checks for the entry with lookup-only and builds and saves only when the key is missing

Key the entry on Cargo.lock and Cargo.toml only, as #6387 does for Linux. The push run then builds once per dependency change and otherwise finishes in about a minute.

Additional context

This should land after #6387. The macOS entry adds about 1.4 GB, and the cache budget has no room for it until #6387 frees some.