added nixos support | removed Cargo.lock from .gitignore

[sandptel]

#99

NixOS/nixpkgs#363853

[eirnym]

Why have you removed Cargo.lock from gitignore?

[sandptel]

Hi @eirnym,
It's been a while and I do not exactly remember why I chose to do that ( mostly because the nix builder need to calculate the sha256 hash of the dependency graph of rust/cargo libraries used here, by readily fetching all of those and then calculating that and matching it with the mentioned one in the derivation to be sure about the reproducibility of the following package). But after a proper review a working derivation of vtracer has been merged in nixpkgs and is readily available to use there, which removes the need for the following addition.