Sounds like a great idea! Unfortunately, pretty much everything is soldered to the motherboard in the Pixel 2013. Even the RAM and SSD is non-upgradable for this same reason. In general, most laptop CPU’s are non-upgradable, as they are soldered to the motherboard nearly 100% of the time.
If you’re dead-set on upgrading to a machine with a better CPU, I would suggesting selling the Pixel 2013 and putting the money towards a used MacBook Pro Unibody with an i7. This way you can also upgrade the Mac’s hard drive to an SSD or even a dual-SSD set-up. The best part about the MacBook Pro Unibody Models: even ones that are in really bad shape can be purchased for cheap, and then flipped into really nice laptops using replacement parts here on iFixit.
I actually bought a MacBook Pro (2011) running an i7 for $400, then installed two 120GB SSD’s by using the dual-drive part (Unibody Laptop Dual Drive) and then installed Windows 10 on the second drive. Now, I have a really solid MacBook Pro running an i7, with an SSD, PLUS, another SSD running Windows 10 for troubleshooting Windows machines for clients. I would strongly consider that route.