Add nix flake with correct versions

[zebreus]

It is currently quite difficult to compile this project, because it only works with llvm 7 and an outdated glibc.

This PR adds a nix flake that makes it easy to get the dependencies at the correct versions. If the nix package manager is installed, nix develop . can be used to enter a reproducible build environment.

I also updated catch2, because the vendored header required an old glibc version.