ci-build.yml: Cargo cache key references a step id that does not exist, so it never invalidates on a toolchain bump #243

Also found while modelling ddx's wheel CI on this workflow.

ci-build.yml L46 keys the Cargo cache on the toolchain:

      - uses: dtolnay/rust-toolchain@stable

      - name: Cache Cargo
        uses: actions/cache@v4
        with:
          key: cargo-cache-${{ steps.rust-toolchain.outputs.cachekey }}-${{ matrix.platform.runner }}-...

but no step carries id: rust-toolchain, so steps.rust-toolchain.outputs.cachekey resolves to the empty string and the key collapses to cargo-cache--<runner>-<target>-<Cargo.lock hash>.

The cache still works, so nothing fails — but it no longer invalidates when the toolchain moves. @stable rolls forward roughly every six weeks, and on that day the job restores artifacts built by the previous compiler under a key that claims to describe the new one. Usually harmless, occasionally a confusing rebuild or a stale-artifact error that does not reproduce locally.

One-line fix:

      - uses: dtolnay/rust-toolchain@stable
        id: rust-toolchain

Worth noting the same key is used across five matrix entries that differ in platform.target but share a runner for the two Ubuntu rows (aarch64 and x86_64 both on ubuntu-latest) — those are distinguished by target, so that part is fine.

Happy to send a PR alongside #242 if useful.