Migrate towards a "unified" flake

[willcl-ark]

I' dlike to move this towards more of a single unified flake, something more like https://github.com/willcl-ark/bitcoin/blob/nix-flake/flake.nix ) which stays as nix-native as possible, but includes (a) useful devShell(s), and can build derivations natively.

Currently it's tricky to support depends and nixpkgs builds from the same devshell. This may be impossible to solve without wrappers, path manipulations, or invasive cmake changes in bitcoin/bitcoin. Therefore we will likely want two devShells, one for each build type.

Derivations won't have this issue.

Changes I'd like to land:

• Use "native" gcc env (see Improve flake compiler setup and CI integration #35)
• Fixup to QT (esp. when using wayland)
• Fixups to USDT (currently not detected correctly)
• Adding in build derivations (so nix build) will build properly