ci: Update GitHub action `actions/cache`

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

Changelog is at <https://github.com/actions/cache#v5>.

.github/workflows/build.yml

@@ -25,11 +25,11 @@ jobs:
           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