Cache the native binding build in tests.yml, and use the pinned Rust #1038

In tests.yml, each run-tests matrix leg compiles the @cipherstash/auth and @cipherstash/profile native bindings from scratch before pnpm run test. #1001 added the step, so the tests can load both bindings. freshtonic and coderdan both raised it in review of #1001.

It has no cache, and it ignores the pinned Rust

• The step restores no Rust cache, so every leg compiles both bindings from cold. The protect-ffi step before it restores index.node from a content-hash cache instead.
• Nothing in the job runs mise at the repository root, so the step uses the runner's default Rust, not the 1.94.1 pinned in the root mise.toml. It passes today, because napi build only needs the crates to compile.

Fix

Cache the two built bindings by a content hash of their sources, as the protect-ffi step does, or restore a Rust cache. Run the build under the root mise toolchain, so it uses the pinned Rust.