Desktop: Disable built-in copy of Simple Backup by default in Joplin Portable

[personalizedrefrigerator]

Summary

Disables the built-in version of Simple Backup by default in Joplin Portable.

Rationale

Joplin Portable is intended to be usable on external drives (e.g. thumbdrives/SD cards). These storage media often have low memory and slow write speeds.

(The idea for this pull request was originally discussed with @laurent22 and @pedr.)

Testing

1. Remove Joplin's profile directory (rm -r ~/.config/joplindev-desktop)
2. Start Joplin in dev mode
3. Verify that Simple Backup is not disabled
4. Close Joplin
5. Simulate Joplin portable by running

zsh$ mkdir /tmp/foo
zsh$ export PORTABLE_EXECUTABLE_DIR="/tmp/foo/"

6. Start Joplin in dev mode
7. Verify that Simple backup is disabled
8. Enable Simple Backup
9. Restart Joplin
10. Verify that Simple Backup is now enabled.

This has been tested successfully on Ubuntu 23.10.