chore: bust github action cache

Author: mattjbray

.github/workflows/build.yml

@@ -41,6 +41,7 @@ jobs:
       - name: Use OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v2
         with:
+          cache-prefix: v2
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
           opam-depext-flags: --with-test
           opam-local-packages: |