Disable HTTP 2 in flapak tests on SDK

Following the same approach used at image generation, disable HTTP 2 in flatpak to overcome connections issues as OSTree does not support retries.

• infrastructure/apertis-image-recipes!612 (merged)
• https://github.com/ostreedev/ostree/issues/2570
• infrastructure/apertis-issues#436 (closed)

Signed-off-by: Walter Lozano walter.lozano@collabora.com