Bugfix: Don't preserve hard links in the initrd.

This fixes a bug that stopped my system from booting but my compressed initrd to grows from 6.5M to 6.7M.

The story:

My system stopped booting after a recent nixos-rebuild, with btrfs complaining that a shared library (libz.so.1) had zero length.  When I unpack the initrd with cpio, I indeed see a zero-length file.  Perhaps if I had passed the right argument, the hard link would have been recovered, but the point is that the Linux kernel's cpio also seems to see a zero-length file.

A brief web search showed some mention of the initrd format not allowing hard links, but I did not dig too deeply.

James