prusa-slicer/super-slicer: move NIX_LDFLAGS into env for structuredAttrs

  doInstallCheck = true;

  env = {

    # The build system uses custom logic - defined in

    # cmake/modules/FindNLopt.cmake in the package source - for finding the nlopt

    # library, which doesn't pick up the package in the nix store.  We

    # additionally need to set the path via the NLOPT environment variable.

    NLOPT = nlopt;

  }

  // lib.optionalAttrs withSystemd {

    # prusa-slicer uses dlopen on `libudev.so` at runtime

  NIX_LDFLAGS = lib.optionalString withSystemd "-ludev";

    NIX_LDFLAGS = "-ludev";

  };

  prePatch = ''

    # Since version 2.5.0 of nlopt we need to link to libnlopt, as libnlopt_cxx