feat: Nix support for reproducible builds and dev shell

[vikingnope]

Pull Request Description

This pull request adds comprehensive Nix support for the project, introducing a Nix flake for reproducible builds, a development shell, and automation for updating dependency hashes. The changes enable easier development and packaging on Nix-based systems, automate hash management for dependencies, and improve the developer onboarding experience.

Nix Flake and Build Infrastructure:

• Added a new flake.nix file that defines the project as a Nix flake, specifying package builds, dependency pinning, and custom source builders for media_kit and related plugins. This allows reproducible and declarative builds of the Flutter application on NixOS and compatible systems.
• Introduced custom Nix derivations for the fvp plugin (nix/fvp.nix) and media_kit_libs_linux (nix/media_kit_libs_linux_donutware.nix), ensuring all required native dependencies are fetched and built in a fully reproducible manner.

Development Environment:

• Added a shell.nix file that sets up a complete development environment with all necessary build tools, libraries, and environment variables for Flutter and Linux desktop development. This streamlines onboarding and ensures consistency across developer machines.
• Updated .envrc to automatically use the flake-based environment if Nix is installed, improving developer experience for those using direnv.

Automation and CI:

• Added a GitHub Actions workflow (.github/workflows/update-nix-hashes.yml) that automatically updates Nix hashes for media_kit when pubspec.lock changes, and creates a pull request with the updated hashes. This reduces manual maintenance and keeps builds reproducible.

Issue Being Fixed

N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.

[vikingnope]

I am going to make some changes so that people can use this within their flake.nix.

Sorry I am still getting used to the whole system 😅

[vikingnope]

I had to do some workarounds for fvp (specifically mdk-sdk, I believe there was a discussion to remove this #500), and mimalloc which were not getting fetched automatically

[PartyDonut]

Sorry for the late response.

While I appreciate the implementation and can only assume it works for using this on nix. This would probably only satisfy a small community, but would require a quite a bit of up-keep. Especially given that I have no way of testing it.

Probably better to keep specific install instructions like this in a separate repository.