nix: remove old niv code paths & various fixes #1109
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Description
I was requested to add retesteth to the Nix shell, and I discovered a lot of inconsistency and stale code, so I cleaned it up as best I could.
Proposed Solution
We don't use Niv, so remove all mention of it, pull sources exclusively using the flake, and use flake-compat to pull them out for stable Nix users (like the buildkite).
Important Changes Introduced
The most important thing is to make sure CI still passes since we no longer use some of the old niv pins, and also to make sure I didn't overlook anything. Once it is green, it should be ready to merge.