Fix: distinguish MSYS2 from Cygwin

[jonahbeckford]

The original code looked like it had a bad merge at some point in its history (duplicates the if condition):

            if (OpamString.ends_with ~suffix:"cygwin1.dll" tx ||
                OpamString.ends_with ~suffix:"msys-2.0.dll" tx) then
            (* ... *)
            else if OpamString.ends_with ~suffix:"msys-2.0.dll" tx then

Without this PR, MSYS2 uses cp -PRr instead of the intended rsync; rsync is a reasonable fallback for the lack of symlinks in MSYS2, and it copies binaries correctly (ex. if both dune and dune.exe are in the same directory, cp will fail).

The PR does not affect Cygwin which by default uses symlinks.

This PR is the real fix to https://github.com/ocaml/dune/blob/fa3aa2891ac74828a81f73894fe247f69c530c81/ci/build-test.sh#L82-L94 and similar failures.

[dra27]

Thanks! Possible new location for the deleted comment 🙂

[dra27]

LGTM, thanks!