libutil: use uniform initialization in _deletePath

Otherwise I get a compiler error when building for NetBSD:

src/libutil/util.cc: In function 'void nix::_deletePath(const Path&, uint64_t&)':
src/libutil/util.cc:438:17: error: base operand of '->' is not a pointer
  438 |     AutoCloseFD dirfd(open(dir.c_str(), O_RDONLY));
      |                 ^~~~~
src/libutil/util.cc:439:10: error: 'dirfd' was not declared in this scope
  439 |     if (!dirfd) {
      |          ^~~~~
src/libutil/util.cc:444:17: error: 'dirfd' was not declared in this scope
  444 |     _deletePath(dirfd.get(), path, bytesFreed);
      |                 ^~~~~