command-not-found: Add explicit fish shell integration

Fish completely separate of nixos will look for command-not-found
provided by nixos, this was surprising so lets make it explicit.

Changed files
+7
nixos
modules
programs
command-not-found
+7
nixos/modules/programs/command-not-found/command-not-found.nix
···
}
'';
+
# NOTE: Fish by itself checks for nixos command-not-found, let's instead makes it explicit.
+
programs.fish.interactiveShellInit = ''
+
function fish_command_not_found
+
"${commandNotFound}/bin/command-not-found" $argv
+
end
+
'';
+
environment.systemPackages = [ commandNotFound ];
};