Speed up evaluation of grids by evaluating custom packages once.

The current code evaluates our custom packages once for each node, which adds
signifcant amount of time to evaluate a grid. We can reduce this, by adding the
custom package set as an attribute to the nixpkgs set we pass to morph.

This doesn't change how we refer to those packages, as we continue to expose the
custom package set as a module attribute.

These are the times to evaluate all three grids (on a partially loaded system),
when there was nothing new to build:

```
before:
real	2m27.837s
user	3m35.528s
sys	0m3.722s

after:
real	1m12.748s
user	1m34.047s
sys	0m3.346s
```