ci: update actions/cache@v2 to v4

Author: Xyene

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: ~/.opam
           key: ${{ matrix.os }}-opam-${{ matrix.ocaml-version }}-flambda-musl-v6