use opam conf-time to install Windows depa

setup-ocaml no longer exposes cygwin

.github/workflows/coq-windows.yml

@@ -47,17 +47,9 @@ jobs:
     - run: opam pin add ocamlfind 'git+https://github.com/JasonGross/ocamlfind.git#win32-fix'
     - run: opam pin add --kind=version coq ${{ env.COQ_VERSION }}
     - run: opam install js_of_ocaml
-
-    - name: Install System Dependencies
-      run: |
-        %CYGWIN_ROOT%\setup-x86_64.exe -qnNdO -P time,zip
-      shell: cmd
-
+    - run: opam install conf-time
     - name: Work around https://github.com/actions/checkout/issues/766
-      run: |
-        %CYGWIN_ROOT%\bin\bash.exe -l -c 'git config --global --add safe.directory "*"'
-      shell: cmd
-
+      run: opam exec -- git config --global --add safe.directory "*"
     - name: echo build params
       run: .\etc\ci\describe-system-config-win.ps1
     - name: deps