attempts to fix the nightly and weekly tests

as per a workaround suggested in ocaml/setup-ocaml#678

Author: ivg

.github/workflows/nightly.yml

@@ -36,6 +36,7 @@ jobs:
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
           dune-cache: ${{  matrix.os != 'macos-11' }}
+          opam-pin: false
           opam-disable-sandboxing: true
           opam-repositories: |
             default: git+https://github.com/ocaml/opam-repository.git

.github/workflows/weekly-regress.yml

@@ -25,6 +25,7 @@ jobs:
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
           dune-cache: true
+          opam-pin: false
 
       - name: Install System Dependencies
         run: opam depext -y bap