fix(ci): unbreak macOS by disabling rust-cache bin caching

[pratyush618]

Summary

The Python Tests (macos-latest / Python 3.13) job has been failing on every master push since the Swatinem rust-cache action's saved ~/.cargo/bin/ got poisoned (master run 25851399868, master run 25845951470). uv sync --extra dev runs maturin → cargo metadata, and macOS responds with:

error: error: unexpected argument 'metadata' found
Usage: rustup-init[EXE] [OPTIONS]

i.e. the cached cargo proxy is dispatching as rustup-init.

Root cause

Two upstream behaviours compose into the failure:

1. rustup 1.28 turned the toolchain installer into a multicall binary — cargo, rustc, rustup, rustup-init are hardlinks/symlinks to the same executable, dispatching by argv[0].
2. Swatinem/rust-cache@v2 caches ~/.cargo/bin/ by default and runs a "cache clean" pass that mutates the directory. Under the rustup 1.28 layout, restoring the cached bin dir overwrites the freshly-installed multicall binary with a stale rustup-init, breaking the cargo proxy.

The action's README documents this exact failure mode under "Known Issues" and recommends cache-bin: false as the fix. References:

• Swatinem/rust-cache README — Known Issues
• Swatinem/rust-cache #16 — cargo and rustup get deleted when saving cache
• rust-lang/rustup #4239 — cargo proxy has corrupted arg0 for rustup v1.28

Fix

Two inputs added to every Swatinem/rust-cache@v2 invocation in .github/workflows/ci.yml (5 jobs: lint, rust-test, rust-test-postgres, rust-test-redis, test):

• cache-bin: false — stop caching ~/.cargo/bin/. dtolnay/rust-toolchain@stable reinstalls the proxies in <5 s on every run; there is no meaningful cache-hit benefit from including the bin dir, and the failure class is eliminated.
• prefix-key: v1-rust-bin-fix — bumps the cache key so the existing v0-* caches (which contain the poisoned macOS bin dir) are evicted. Without this the very next run would still restore the broken cache before the new cache-bin: false directive applies on save.

Applied to all five call sites (not only the macOS-affected test job) for consistency and to immunize the Linux/Windows runners against the same regression when their runner images rotate to rustup 1.28.

The valuable caches (target/, ~/.cargo/registry/, ~/.cargo/git/db/) are unaffected.

Test plan

• PR CI must pass on macOS: Python Tests (macos-latest / Python 3.13) and Python Tests (macos-latest / Python 3.10) complete uv sync --extra dev, the maturin build, and pytest tests/ -v.
• Linux + Windows matrix entries still pass, no regression in timings.
• After merge, the next master push CI run is fully green.
• gh cache list --ref refs/heads/master shows new entries starting with v1-rust-bin-fix-… after the post-merge run.