generate-cabal-file.hs: add --release command-line flag to build cabal2nix only
generate-cabal-file.hs: add --release command-line flag to build cabal2nix only

hackage2nix should be used only from the Git repository.

spencerjanssen/cabal2nix,psibi/cabal2nix,dmalikov/cabal2nix,Fuuzetsu/cabal2nix,bennofs/cabal2nix,jb55/cabal2nix,gridaphobe/cabal2nix