There is one simple solution to this thing, I have not experimented on Google Pixel.
If the Install later option is showing then you can choose the option to stop the software from being upgraded or the download files will be present in the download folder. From thereafter file gets downloaded you can go there and delete the file before installing it.
I have ...
Reboot into bootloader/fastboot mode.
$ fastboot getvar all | grep current-slot
$ fastboot --set-active=a
$ fastboot reboot
You can set the active slot to be a or b depending on which slot is currently used, from the output of the first command. For example, if you see (bootloader) current-slot:b, you set a as active instead. You can rerun the first command ...