install-nix-from-closure improvements

Just wasted a couple hours chasing shadows because the nix store got corrupted and there was no indication of that anywhere.

Since an install is one-time only, might as well verify and optimize.
The optimization also showed that the copied files aren't read-only; fixed that as well.

On closer inspection the 2004 IEEE spec does say that pipelines can be preceded with a `!`. http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_09_02

I'll revert the `!` things for legibility.