Add NixOS Flake

[theoparis]

Based on #154, this PR adds a nix flake.
The changes I've made compared to #154 are:

• Signed commits
• Added a Nix derivation
• Added support for other systems (e.g. riscv64) via nix-systems
• Used nixfmt (nixfmt-rfc-style package) for formatting

To Do:

• Fix building on Darwin
• Add runtime dependencies so you can nix build .# && ./result/bin/redot

Darwin support is still WIP which is why I made this a draft. It seems like AppKit/AppKit.h is not being found despite it being present in buildInputs.
Additionally, I had to temporarily modify platform_methods.py to allow finding moltenVK in ${darwin.moltenvk}/lib/libMoltenVK.dylib. This could probably be improved with a proper lookup of the entire sdk directory instead of looking in lib or other hard coded paths 🤔

Edit: Hmm, AppKit.h is not in an AppKit/ directory an is instead just AppKit.h via something like /nix/store/0nh8rqwsdaj9pj6lw0nkggry8ykkpazg-apple-framework-AppKit-11.0.0/Library/Frameworks/AppKit.framework/Versions/C/Headers/AppKit.h.

[Spartan322]

Is this still being worked on, and has this been proposed to Godot?

[Spartan322]

Also does this close #154?

[MichaelFisher1997]

@Spartan322 I have my eye on this.
#154 is mine, this is an improvement on that (so probs close #154)
its gone a bit far past my knowledge of nix shells, but a few people in the community are asking for it.
ive spoken to a few people, we will be getting back to this, a few use it already to do local builds

[MichaelFisher1997]

Is this still being worked on, and has this been proposed to Godot?

yes, no