I'm posting this should anyone face the same issue of the installer failing due to being unable to access the \Affinity\Photo folder.
When upgrading to 1.7.0.367 and again just now to 1.7.1.404 the installation failed because even I, as a windows 10 administrator, could not access (view contents or delete) the \Affinity\Photo folder. I could not take control of the folder and the first time spent a few hours finding what appeared to be an operating system issue and found an unexpected solution: by sharing the \Affinity\ folder as owner and rebooting, I could delete both folders and successfully install Photo. (note: this has been the only folder group I have ever had this issue with as far back as Windows1.1 ;-)