Hardcoded temp path prevents multiple users using Inky on one system

Inky uses a directory `/tmp/inky_compile` for temporary files to compile.

Because this is a hard coded path and includes no random element, it is the same for any user on the system, but it is owned by the first user on a system to run Inky, and it's permissions are that other users don't have write access.

This means that if any user of a system has run Inky recently, all other users will receive an error message when loading Inky, and be unable to preview their story.