html-tidy: fix on mingw
html-tidy: fix on mingw

This doesn't fix the cross-build by itself due to
https://github.com/NixOS/nixpkgs/issues/14965
But in this case one can hack around that "easily" by:
  (html-tidy.override {
    inherit (/*non-cross nixpkgs eval*/)
      cmake;
  }).crossDrv

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