Since you're on Fi you have an 'in' with Google. Contact their customer support through the Settings application and they should help you, or at least give you a satisfactory answer as to why you're not getting the latest Security Update.
Ha! I contacted them thru chat last night. Went thru all the motions of rebooting phone, switching carrier to Sprint, then T-Mobile, checked for updates, and no update. The tech says he has to kick the situation up to someone else.
Then I got this email from Fi Support: "Thank you for contacting Project Fi Support. This is Erica, and I will do everything in my power to resolve your concern today. I understand you're inquiring about the 8.0 Android update for your Nexus 6P, and I can see how that will pose an inconvenience. At this point, there is not an official date for when any device will get the 8.0 update. However, once the update is ready for your device, you'll get a notification to download and install the update."
So, no satisfactory reply from Project Fi.