use ~ instead of $HOME for cache path

Author: jchavarri

.github/workflows/playground.yml

@@ -14,15 +14,17 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2.0.0
       - name: Cache .opam folder
+        id: cache-opam
         uses: actions/cache@v1
         with:
-          path: $HOME/.opam
-          key: ${{ runner.os }}-opam-v1
+          path: ~/.opam
+          key: ${{ runner.os }}-opam-4.06.1-3.5.1-v1
       - name: Use OCaml ${{ matrix.ocaml-version }}
         uses: avsm/setup-ocaml@v1.0
         with:
           ocaml-version: ${{ matrix.ocaml-version }}
-      - run: opam install js_of_ocaml.3.5.1
+      - if: steps.cache-opam.outputs.cache-hit != 'true'
+        run: opam install js_of_ocaml.3.5.1
       - run: git submodule update --init && ./scripts/buildocaml.js
       - run: ./scripts/ninja.js config && ./scripts/ninja.js build
       - run: mkdir playground && mkdir playground/stdlib