fix(fish-speech): allow invalid_reference_casting so tokenizers builds on darwin

[localai-bot]

Failure

In the v4.5.5 release CI, the fish-speech backend image build job FAILED on darwin/metal arm64 (mps profile). All Linux variants (cpu/cublas/etc) built fine.

The failure happens during the editable install of fish-speech source (pip install --no-build-isolation -e ... in backend/python/fish-speech/install.sh):

error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
error: could not compile `tokenizers` (lib) due to 1 previous error; 8 warnings emitted
error: `cargo rustc ... --crate-type cdylib ...` failed with code 101
ERROR: Failed building wheel for tokenizers
Failed to build tokenizers
make[1]: *** [fish-speech] Error 1

Root cause

On darwin arm64 the transitive tokenizers Python package compiles its Rust extension from source, because there is no prebuilt manylinux wheel for that platform. (On Linux the prebuilt wheel is used, so the Rust code never compiles there - which is why only the darwin job broke.)

The pinned tokenizers crate that fish-speech's dependency stack resolves to contains a &T -> &mut T cast. That pattern is flagged by the invalid_reference_casting lint, which is deny-by-default in the macOS CI runner's newer Rust toolchain. It compiled under the older toolchain but is now rejected.

Fix

Export RUSTFLAGS with -A invalid_reference_casting before installRequirements in backend/python/fish-speech/install.sh, so the unchanged third-party crate compiles as it did before the toolchain bump. The flag is appended to any existing RUSTFLAGS rather than clobbering it.

This is version-agnostic (no risky dependency repin) and harmless on Linux, where no Rust compile happens.