Document how to use sweettest in Holonix

[ThetaSinner]

When you get Holochain/Lair through Holonix, the builds are defined so that you get the right libraries provided on supported platforms.

However, if you use sweettest then you are going to have to rebuild most of the code for both, so you actually need to provide the same Nix packages in your dev shell to run those tests.

We have an example of how this is done over here https://github.com/holochain/tryorama/blob/main/flake.nix#L42

Should we provide these libraries for everyone so this just works/ Ideally not because that's a lot of extra bandwidth for most projects that don't use sweettest.
Or should we just document what to change in your flake if you want to use sweettest? I prefer that option