I had this same issue. I can pretty much promise you unless you do what I suggest, you'll never get an OTA because you're running prerelease firmware. I know this because I was in your very same position. My New Nexus 6 came in with that very same build. It has something to do with the prerelease firmware and checking against your carrier for firmware. Yes the OTAs come from google but, there is also a check to see if the OTA is appproved by the carrier. This was new for the Nexus 6. The good news is there is that there is a OTA to get you out of the prerelease firmware. Yay! Bad news is...You'll have to remove your sim and factory reset the phone. Removing the sim and powering the phone on and off will not work, like some suggest. The phone will remember the last sim/carrier you used... Here is what I did.
Removed my sim. Factory reset the phone. During the setup it will ask you to insert a sim. Just skip ahead and setup the phone on your wifi without a sim...... I wouldn't even bother setting up your google account on the phone because I would suggest another factory reset at the end of this process. Once you get the phone setup and on wifi check for updates. You should now be getting ota's. Rinse and repeat with the updates. You'll probably be updated all the way to Marshmallow. Once you're offered no more ota's, insert your sim and factory reset the phone again. You're done.