clean up flake.nix

[figsoda]

• nixpkgs-mozilla -> fenix: it is more flake friendly and more actively maintained
• mkDerivation -> buildRustPackage: this is the nixpkgs way of building rust packages without, it also replaces import-cargo
• enables tests: we don't have tests right now but we can catch problems like #11, also integrates well if we are going to add github actions in the future. buildRustPackage will pick up rust tests unlike mkDerivation
• reads version from bin/Cargo.toml so it doesn't have to be updated every time (vim plugins still needs version updates)
• updates rust channel: because we switched to fenix, the previous cache wouldn't work any more, so why not

[oppiliappan]

reads version from bin/Cargo.toml so it doesn't have to be updated every time (vim plugins still needs version updates)

This is amazing, thank you!

[oppiliappan]

Made some minor re-arrangements to the flake, to remove rec and a statix warning.