nixos-rebuild switch --upgrade, and that broke Vivaldi for me (some pages just die for whatever reason). So I rebooted into the previous config, pre-upgrade, and run
/run/current-system/bin/switch-to-configuration boot to make it current.
Now when I boot, the previous configuration is selected as a default. However, the newer, broken configuration is still there, and gives me aesthetics pain How do I remove it from boot menu?