pkg-config not resolved on windows with dune configurator

Expected Behavior

Once pkg-config --libs libpng returns the value get it and return from C.Pkg_config.get

Actual Behavior

C.Pkg_config.get on windows always return None

Reproduction

1. Use windows server for example in GitHub actions

jobs:
  build:
    runs-on: ${{ matrix.os }}
    strategy:
      matrix:
        os: [windows-latest]
        # os: [ubuntu-latest, macos-latest, windows-latest]
    steps:
      - uses: actions/setup-node@v2
        with:
          node-version: '13'
      - uses: actions/checkout@v2.3.2

      - name: Install libpng
        run: |
          C:\vcpkg\vcpkg install libpng
          ls C:\vcpkg\packages\libpng_x86-windows\lib\pkgconfig
          $env:PKG_CONFIG_PATH += 'C:\vcpkg\packages\libpng_x86-windows\lib\pkgconfig'
          pkg-config --libs libpng # returns correct flags
        #..prepare your env 
       - name: Run dune
         run: |
          dune build -p myprojec

With a simple dune configurator executable

(executable
 (name discover)
 (libraries dune-configurator))

And discover.ml code

module C = Configurator.V1

let () =
C.main ~name:"odiff-c-lib-package-resolver" (fun c ->
let default : C.Pkg_config.package_conf =
  { libs   = ["libpng"]
  ; cflags = []
  }
in
let conf =
  match C.Pkg_config.get c with
  | None -> default
  | Some pc ->
     match (C.Pkg_config.query pc ~package:"libpng") with
     | None -> default
     | Some deps -> deps
in

Printf.printf "Resolved c flags %s" (String.concat "" conf.cflags);
Printf.printf "\nResolved c libs %s" (String.concat "" conf.libs);

C.Flags.write_sexp "c_flags.sexp"         conf.cflags;
C.Flags.write_sexp "c_library_flags.sexp" conf.libs) 

Observe that conf.cflags and conf.libs are always empty

Specifications

• Version of dune (output of dune --version): 2.7.1
• Version of ocaml (output of ocamlc --version): 4.11.1
• Operating system (distribution and version): Windows Server 2019 (easy to reproduce on Github actions)

Start with the dune-configurator implementation behind C.Pkg_config.get and reproduce the issue using the discover.ml example on Windows Server 2019 or windows-latest. Compare the result with the pkg-config command shown in the GitHub Actions workflow; done means C.Pkg_config.get and query return the libpng flags instead of empty values on Windows.