nixos: Fix sysctl option merging

Using pkgs.lib.mkOverride in a sysctl option would throw a bogus error.

Also, if you defined a sysctl multiple times in the same configuration,
only one of the values would be picked up, while the others were silently
discarded.

This patch should fix both issues. If you define a sysctl multiple
times at your highest defined priority level, you will get a proper
error with detailed location information.