build: fix builds on Nix

[saikatdas0790]

This is a draft PR that updates the Nix build system to use a recentish commit from Nixpkgs that has more recent versions of Rust Nightly. Specifically rustc 1.75.0-nightly (a2f5f9691 2023-11-02)

It also moves away from using Nix flakes and uses Nix shell for the following reasons:

• Nix flakes are still experimental while Nix shell is stable and mature tooling
• The current Nix flake uses a Rust overlay which is an additional component that needs to be added. This pull request uses native cargo tooling and puts the toolchain specific particulars in a rust-toolchain.toml file
• The default.nix pins the dependencies to a specific commit ensuring that build dependencies aren't automatically updated when the upstream repositories get updated. The commit is the most recent commit that passes all tests from the Nixpkgs repo. Future updates just need to bump the commit version to another recent version that passes all tests

Performing a couple of additional checks before this is ready to merge