fix(nix): resolve issues and revamp the flake

[water-sucks]

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Description

This fixes up some inconsistencies and regressions with the Nix flake in this repository.

• The Go version used by nixpkgs was 1.24, while the current required toolchain version is 1.25.
• The vendor hash was wrong due to upgraded dependencies.
• Filtering out files was super clunky and didn't scale with the gitignore, so this uses a special nixpkgs function that automatically filters ALL files found in .gitignore transparently.
• The version was not synced with the latest version, and was stuck on 1.4.2 while the latest at time of writing is 1.6.0.

Additionally, I decided to follow Nix best practices and avoid antipatterns such as with, as described in https://nix.dev/guides/best-practices; instead, inherit is used whenever possible.

I also took the liberty of adding myself as a maintainer for the Nix package, and will likely do so in nixpkgs as well for future updates, since over there the Nix version is still stuck on 1.5.2, rather than the latest 1.6.0, again at time of writing.

How to test?

Build the Nix package using nix build, and also drop into the Nix devshell using nix develop to make sure the correct version of Go and all other Go SDK tooling is still available.

[oschwartz10612]

LGTM