[CSL-1509] Add 'pkgs/stack2nix' to '.gitignore'
[CSL-1509] Add 'pkgs/stack2nix' to '.gitignore'

input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl