Only run Cygwin setup once during opam init

[dra27]

In opam 2.2.x, the only way to get an MSVC compiler at opam init is to manually specify the compiler or to have an MSVC compiler in PATH so that ocaml-system enables it. Otherwise, a mingw-w64 GCC package will be required via depext in order to install the mingw-w64 port of OCaml.

At present, with the internal Cygwin installation enabled, Cygwin setup is invoked at the start of opam init on Windows to perform the initial installation and is then invoked again at depext resolution.

It would be good to coalesce these into a single call to setup, given that it makes the OOBE (out-of-box-experience) even slower. It's not at present trivial though, because the Cygwin installation may itself be required to get the initial repository (but some of the work in progress with ocaml-tar, etc. may mean this becomes unnecessary, or at least not the default, in future).