[Enhancement] Cache TTL environment variable

[Vitriia]

Not a bug but it can be a real issue :

The way backup are actually implemented in Yunohost actually have a little loophole : for installations with a lot a small apps and some big apps, full backup can take for ever. It's because Borg cache have a Time To Live set by default to 20 backups (same machine & user).
So if you have 8 apps borg will recalculate each chunk every 2 full backup (8 apps + conf & data).
It's far worst if the installation have more than 18 apps because chunks will be recalculated each full backup.

A way to prevent it is to integrate BORG_FILES_CACHE_TTL environment variable has a yunohost app setting.
This way, admins who want to custom this setting can do it in a flexible way (aka not having to change the backup_method each time borg app is updated).

The dedicated environment variable doc
The Cache TTL explained

Have a nice day