Simplify flake

[bohendo]

Now that crytic.nix is public, we don't need to define nix config for solc-select, crytic-compile, etc in this repo. Instead, we can use the packages provided by crytic.nix.

[bohendo]

Sprinkled a bit of regex into flake.nix to dynamically read the version from cmd/root.go, a couple of implications:

• This assumes that the version constant defined in cmd/root.go is and remains the source of truth.
• it hardcodes the regex: .*const version = "([^"]+)".*, this would need to change if eg we rename this variable, switch to single quote raw strings, etc

[anishnaik]

I've actually decided that i'm a little wary of using the regex for the reasoning you mentioned. I've added it to the release checklist and we can just manually update it every time we cut a release :)