Determinate Nix 3.0 featuring stable flakes

Lots of exciting things here:

- Formally stabilizing flakes has been long awaited by everyone, I think.

- Parallel evaluation will improve developer experience - evaluation speed seems to be at the top of the list of feedback I've received from colleagues whom I've invited to try Nix.

- I'm hoping lazy trees will provide a better experience for flakes in monorepos.

Unfortunately I haven't had a great experience with the Determinate Nix installer when I tried it, though that was a while ago (shortly after launch) so may warrant revisiting.

I'm also concerned about the growing schism between Determinate Systems and the Nix community, as can be seen in the Discourse thread. I think there are opportunities to strengthen that bridge, e.g. naming things perhaps such that it's not possible to misinterpret this announcement as "Nix 3.0".

I am also curious to know what is your strategy for upholding the flakes stability guarantee without forking Nix. I'm not sure what the governance structure or roadmap is of the community Nix project, but would it not be possible that the project would want to eventually introduce a breaking change (e.g. to revisit the cross-compilation or parameterization aspects) that would affect Determinate Nix users?