Do not keep avatars cache in the backups

[kuba-orlik]

Describe the bug

When backing up the app, the archive is filled with cache/avatars and cache/headers files. This can be tens of gigabytes and is just essentially just cache that can be rebuilt. Keeping it in archive makes the backup and upgrade process take literal hours.

Context

• Hardware: VPS bought online
• YunoHost version: 11.3.0.2
• I have access to my server: Through SSH
• Are you in a special context or did you perform some particular tweaking on your YunoHost instance?: no
• Using, or trying to install package version/branch: all versions are affected

Steps to reproduce

• Perform a backup of the app
• Open the archive
• See tens of gigabytes of cached images from other instances

Expected behavior

The cached images from other instances are not included in the archive