conf-findutils: add MSYS2 depext declaration

[smorimoto]

The conf-findutils package currently has depext declarations for Cygwin but is missing one for MSYS2:

depexts: [
  ["system:findutils"] {os = "win32" & os-distribution = "cygwinports"}
  ["findutils"] {os-distribution = "cygwin"}
  # Missing: ["findutils"] {os-distribution = "msys2"}
]

On MSYS2, findutils is available as the findutils package via pacman, so the fix should be straightforward:

["findutils"] {os-distribution = "msys2"}

This was discovered whilst adding MSYS2 support to ocaml/setup-ocaml. Without this declaration, packages that depend on conf-findutils cannot have findutils automatically installed via depext when using opam with an MSYS2 environment.

Related: #29575 (conf-m4), #29576 (conf-perl)