Switch pipelining metadata action to hollow rlib (-Zno-codegen)

[Walter-Reactor]

Replace the --rustc-quit-on-rmeta / .rmeta approach with Buck2-style hollow rlibs: the RustcMetadata action runs rustc to completion with -Zno-codegen, emitting a .rlib archive (named -hollow.rlib) that contains only metadata and optimized MIR.

Key fixes:

• Main Rustc action uses full rlib --extern deps so the SVH embedded in the full rlib matches what downstream binaries expect (avoiding E0460 with non-deterministic proc macros).
• RUSTC_BOOTSTRAP=1 is set on both actions: it changes the crate hash, so inconsistent use would cause SVH mismatch even for deterministic crates.
• -Ldependency= points to the _hollow/ subdirectory so the full rlib and hollow rlib never appear in the same search path (avoids E0463).
• transitive_metadata_outputs are always included in the sandbox so rustc can resolve transitive -Ldependency= references.

Adds an SVH mismatch test that demonstrates the problem with non-deterministic proc macros (uses a HashMap-based derive macro whose output varies by OS-seeded random seed).

I've been using this locally, and it seems to be a notable improvement - I've been able to remove all disable_pipelining=True attributes and enable pipelining globally while still having good performance

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