Windows: opam fails to install conf-libcurl.

[yosefAlsuhaibani]

related: ocaml/opam-repository#26894.

Using setup-ocaml@v3 on a windows GHA run: trying to install conf-libcurl fails with the error /usr/bin/sh: line 1: curl-config: command not found. SSH-ing into the runner (w/https://github.com/mxschmitt/action-tmate
) I was able to find curl-config under /d/cygwin/usr/x86_64-w64-mingw32/sys-root/mingw/bin, which makes me believe that setup-ocaml isn't configuring the PATH correctly.

I've reproduced this here: yosefAlsuhaibani/test-ocaml-setup-windows#4, where in the latest commit this run fails (https://github.com/yosefAlsuhaibani/test-ocaml-setup-windows/actions/runs/11978037743/job/33397219358) which is done by this workflow (https://github.com/yosefAlsuhaibani/test-ocaml-setup-windows/actions/runs/11978037743/workflow).