Use setup-ocaml@v2 for lower-bounds CI

opam-0install is incompatible with opam 2.2

Author: sim642

.github/workflows/unlocked.yml

@@ -105,7 +105,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up OCaml ${{ matrix.ocaml-compiler }}
-        uses: ocaml/setup-ocaml@v3
+        uses: ocaml/setup-ocaml@v2
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}