Add a way to cache packages for jupyter lite build

[paddymul]

Problem

Every time I run jupyter lite build mamba/conda downloads a bunch of packages. This is slow, and incompatible with offline development.

Proposed Solution

How do we make conda/mamba read from local caches instead of downloading everything fresh off of the internet.

It looks like hatchling builds projects in some type of ephemeral environment under /var/folders that doesn't have access to system wide caches.

How can we plumb the options in so that we get faster builds?

Additional context