nix-copy-bar: don't exit 1 when there was nothing to copy

common.nix

@@ -42,7 +42,10 @@ let
              drawn=1 ;;
         esac
       done
-      [ -n "$drawn" ] && printf '\n'
+      # `if`, not `[ … ] && …`: with nothing to copy the bar never draws, and a
+      # bare test as the last statement would exit 1 and break the caller's
+      # && chain. A real copy failure still surfaces via the caller's pipefail.
+      if [ -n "$drawn" ]; then printf '\n'; fi
     }
   '';