Update CI cache action to actions/cache@v5

[Copilot]

GitHub Actions is deprecating Node 20 on runners, and actions/cache@v4 depends on that runtime. This updates CI to use actions/cache@v5 so the workflow remains compatible with the runner runtime migration.

• Workflow compatibility update

  • Bumped cache action in .github/workflows/ci.yml from actions/cache@v4 to actions/cache@v5.
  • No changes to cache paths or cache key strategy.

• Behavioral impact

  • Preserves existing Cargo cache behavior while moving to the supported action runtime.

- name: Cache cargo crates
  uses: actions/cache@v5
  with:
    path: |
      ~/.cargo/registry
      ~/.cargo/git
      target
    key: ${{ runner.os }}-${{ matrix.BUILD_TARGET }}-cargo-${{ hashFiles('**/Cargo.lock') }}