Do not run after update

[yara-blue]

Remarkable uses two partitions for the OS and switches between them. One effect of this is that the systemd service use by book-safe is deleted on update. This is kinda useful as it means a user gets the time to backup before enabling book-safe again.

Though this behavior can change in the future.

Proposal:
hash the xochitl executable and if it changes exit with error on scheduled runs. Maybe add a PDF to the users docs informing them?