nixos: Fix iso_graphical evaluation

I broke it:
in job ‘nixos.iso_graphical.x86_64-linux’:
The option `services.udisks2.enable' has conflicting definitions, in `/nix/store/bwcjw1ddj94q83vbbnq1nnrs5aisaw59-source/nixos/modules/profiles/installation-device.nix' and `/nix/store/bwcjw1ddj94q83vbbnq1nnrs5aisaw59-source/nixos/modules/services/x11/desktop-managers/plasma5.nix'.