set: Remove a broken array read

Hardcode "--erase" as a hack, because "argv[-1]" is *not* a thing.

Introduced in aacc71e.

Fixed in master by no longer being C++.

src/builtins/set.cpp

@@ -244,7 +244,7 @@ static int validate_cmd_opts(const wchar_t *cmd, const set_cmd_opts_t &opts, int
  }
 
  if (argc == 0 && opts.erase) {
- streams.err.append_format(BUILTIN_ERR_MISSING, cmd, argv[-1]);
+ streams.err.append_format(BUILTIN_ERR_MISSING, cmd, L"--erase");
  builtin_print_error_trailer(parser, streams.err, cmd);
  return STATUS_INVALID_ARGS;
  }