CI: restore win build

Author: hhugo

.github/workflows/build.yml

@@ -86,7 +86,7 @@ jobs:
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
           opam-repositories: |
-            dra27: https://github.com/hhugo/opam-repository.git#for-jsoo-ci-windows
+            dra27: https://github.com/dra27/opam-repository.git#windows-5.0
             default: https://github.com/ocaml-opam/opam-repository-mingw.git#sunset
             opam: https://github.com/ocaml/opam-repository.git
           dune-cache: true