fix CI

Author: yforster

.github/workflows/build.yml

@@ -12,9 +12,9 @@ jobs:
         uses: actions/cache@v2
         with:
           path: "~/.opam"
-          key: opam-${{github.base_ref}}-${{github.ref}} 
+          key: opamdir-${{github.base_ref}}-${{github.ref}} 
           restore-keys: |
-            opam--refs/heads/${{github.base_ref}}
+            opamdir--refs/heads/${{github.base_ref}}
 
       - name: Install OCaml
         uses: avsm/setup-ocaml@v1
@@ -27,7 +27,6 @@ jobs:
           fetch-depth: 1
 
       - run: opam repo add coq-released https://coq.inria.fr/opam/released
-      - run: opam repo add coq-core-dev https://coq.inria.fr/opam/core-dev
       - run: opam install . --deps-only --with-doc --with-test
 
       - run: opam exec -- make -j 2 all