yarn: adding missing `meta.mainProgram`

+1
+1
pkgs/development/tools/yarn/default.nix
··· 36 license = licenses.bsd2; 37 maintainers = with maintainers; [ offline screendriver marsam ]; 38 platforms = nodejs.meta.platforms; 39 }; 40 })
··· 36 license = licenses.bsd2; 37 maintainers = with maintainers; [ offline screendriver marsam ]; 38 platforms = nodejs.meta.platforms; 39 + mainProgram = "yarn"; 40 }; 41 })