build.prop got wiped (how to restore?)
Go find and download the stock ROM for your phone and try to extract build.prop from it. Then place it in the correct directory and change its owner and permission.

Factory images for Nexus (and Pixel) phones are always available from Google.

Also there's a similar issue from another website. You can download the ROm but only flash system so that the system partition will be restored. You may have to root again if you're not using a systemless root.