CI: separate cache per ocaml-version

We currently compile 4.12.0 every run since only 4.07.1 is cached.
See #296 (comment).

.github/workflows/tests.yml

@@ -1,5 +1,5 @@
 name: build and run tests
-# https://github.com/avsm/setup-ocaml
+# https://github.com/ocaml/setup-ocaml
 
 on:
   - push
@@ -27,17 +27,17 @@ jobs:
         uses: actions/checkout@v2
 
       - name: Cache # https://github.com/marketplace/actions/cache
-        uses: actions/cache@v2.0.0
+        uses: actions/cache@v2
         with:
           # A list of files, directories, and wildcard patterns to cache and restore
           path: |
             ~/.opam
             _opam
           # An explicit key for restoring and saving the cache
-          key: ${{ runner.os }}-new-${{ hashFiles('goblint.opam.locked') }}
+          key: opam-${{ runner.os }}-${{ matrix.ocaml-compiler }}-${{ hashFiles('goblint.opam.locked') }}
 
       - name: Setup OCaml ${{ matrix.ocaml-version }}
-        uses: avsm/setup-ocaml@v1
+        uses: ocaml/setup-ocaml@v1
         with:
           ocaml-version: ${{ matrix.ocaml-version }}