ISO size differ without an obvious reason

During release process QA, it has been discovered that ISO size differ without an obvious reason:

- 0.19: 906.5M

- 0.20: 897.3M

- 0.20.1: 925.4M
- 0.21: 934.5M

It might be related to the build system used to build the ISO.

Feature Branch: bugfix/6390-dont-include-APT-lists-and-cache-in-the-ISO

Original created by @alant on 6390 (Redmine)

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