flk: don't set out path for nix build

./result is more obvious for users who've used nix build and then we
don't have to worry about file extensions, its already a folder with the
right files.

src/pkgs-lib/shell/flk.sh

@@ -77,8 +77,6 @@ case "$1" in
   "build")
     nix build \
       "$DEVSHELL_ROOT#nixosConfigurations.$2.config.system.build.$3" \
-      -o \
-      "$DEVSHELL_ROOT/builds/$2/$3" \
       "${@:4}"
     ;;