Fix NixOS patching
Fix NixOS patching

Moving the `.nix-deps` has resulted in rpath links being broken and
therefore bootstrap on NixOS broken entirely.

This PR still produces a `.nix-deps` but only for the purposes of
producing a gc root. We rpath a symlink-resolved result instead.

For purposes of simplicity we also use joinSymlink to produce a single
merged output directory so that we don't need to update multiple
locations every time we add a library or something.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust