[Integration]: Rewrite *nix CIs to use nix flakes

[imrn99]

Scope

actions

Type of change

refactor

Changes / proposals

Two main reasons:

• Unifying dev and CI environments is a good way to ensure everything builds as intended.
• I tried running tests on my laptop using cargo nextest instead of the built-in cargo test. Tests run 4x faster using nextest. Installing the subcommand is much easier with a flake (hence why I mention this in this issue)