ci: Update GitHub action `actions/cache` — heartwood

Version 4 uses Node.js 22, which GitHub warns about and suggests to update.

           fetch-tags: true
           filter: 'tree:0'
       - uses: dtolnay/rust-toolchain@stable
       - uses: actions/cache@v4
       - uses: actions/cache@v5
         with:
           path: ~/.cargo
           key: ${{ matrix.os }}-${{ runner.arch }}-${{ hashFiles('Cargo.lock') }}-cargo-home
       - uses: actions/cache@v4
       - uses: actions/cache@v5
         with:
           path: target
           key: ${{ matrix.os }}-${{ runner.arch }}-${{ hashFiles('Cargo.lock') }}-cargo-target