fix(nixos): use correct process name

[kajusnau]

In latest nixos-unstable cosmic-panel (or any cosmic tool for that matter) is no longer wrapped and is running under cosmic-panel.
This essentially reverts the change done in d2623cb

Tested manually on NixOS using an overlay for the cosmic-initial-setup package

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[kajusnau]

Turns out there was an issue with the wrapping of cosmic packages in nixpkgs.
Will be addressed by NixOS/nixpkgs#534694.

[Pandapip1]

We should probably just patch killall and pkill like we have with ananicy-cpp (https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/by-name/an/ananicy-cpp/match-wrappers.patch)