pkgs: diff-flake: fix '--flake-output' handling

I'm surprised that `shellcheck` did not pick up this error, `INPUTS` was
leftover from the previous name of the variable before a refactor...

pkgs/diff-flake/diff-flake

@@ -53,7 +53,7 @@ parse_args() {
                 exit
                 ;;
             -f|--flake-output)
-                INPUTS+="$1"
+                FLAKE_OUTPUTS+=("$1")
                 shift
                 ;;
             -o|--output)