feat: nix package, overlay and devshell use the quickemu FlakeHub flake

[flexiondotorg]

Description

This updates the Nix flake to include the current stable release of Quickemu via the FlakeHub flake and always tracks the Nixpkgs stable branch.

• All future Quickgui releases will now also include the stable release of Quickemu.
• Nix devshell now have the current Quickemu version included, directly from us (upstream) rather than potentially out-of-date releases from Nixpkgs
• The builds tool in the Nix devshell track the 6-monthly stable branch of Nixpkgs for better predictability.

Type of change

• Packaging (updates the packaging)

Checklist:

• I have performed a self-review of my code
• I have tested my code in common scenarios and confirmed there are no regressions
• I have added comments to my code, particularly in hard-to-understand sections