ci: fix cache for gh-pages job

.github/workflows/gh-pages.yml

@@ -11,16 +11,16 @@ jobs:
     steps:
       - uses: actions/checkout@main
 
-      - uses: avsm/setup-ocaml@v1
-        with:
-          ocaml-version: '4.12.0'
-
-      - name: cache opam
+      - name: Cache opam
         id: cache-opam
         uses: actions/cache@v2
         with:
-          path: _opam
-          key: opam-ubuntu-latest-4.12.0
+          path: ~/.opam
+          key: opam-${{ matrix.os }}-${{ matrix.ocaml }}
+
+      - uses: avsm/setup-ocaml@v1
+        with:
+          ocaml-version: '4.12.0'
 
       - name: Pin
         run: opam pin -n .