Create .version

[maydayv7]

This is because in lib.trivial.release the file .version is used which leads to complications when nixpkgs.lib is used with nixosConfigurations

[maydayv7]

CC @blaggacao

[blaggacao]

What is the complication?

I think in such cases, nixpkgs/lib is a better fit.

nixpkgs.lib should have no interface or knowledge of nixpkgs. It is meant to make the agnostic part of nixpkgs/lib available to a wider audience, without having to use nixpkgs proper.

Library functions that implicitly depend on nixpkgs are probably wrong here, but there doesn't seem to be a way to filter them out.

These functions could be moved out of ./lib upstream, or any other way of separating agnostic & non-agnostic library functions into different folders could be employed.