refactor(flake): improve usage

[isabelroses]

changes:

• allow for different system architectures
• separate dev deps into separate file
• gui now functions properly when installing though flake

[shadeyg56]

Hi, I am the original author of the flake. I just started using Nix about 2 months ago so my Nix writing skills are not the greatest haha so I appreciate this

I hadn't noticed the GUI wasn't working since I had the files I needed in /usr/local from messing around with auto-cpufreq before having the flake so thanks for catching that!

I did push a few changes. I removed darwin from being a build system because auto-cpufreq only supports the Linux kernel. Also the icon still wasn't working in the about dialog so I added that to the substitute as well.

The module was still using x86_64-linux so I updated it to inherit the system.

Thanks for your contribution and you'll receive credit in the next release!