Test the new docker-coq-action feature allowing "opam_file" folders

Author: erikmd

.github/workflows/build-coq-demo.yml

@@ -40,7 +40,8 @@ jobs:
       - uses: actions/checkout@v2
       - uses: coq-community/docker-coq-action@v1
         with:
-          opam_file: 'coq-demo.opam'
+          # opam_file: 'coq-demo.opam'
+          opam_file: '.'
           coq_version: ${{ matrix.coq_version }}
           ocaml_version: ${{ matrix.ocaml_version }}