Disable HTTP pipelining in APT to avoid "Hash Sum Mismatch"

Disable HTTP pipelining in APT to avoid "Hash Sum Mismatch"

Work around issues with the APT downloader corrupting files and causing "Hash Sum Mismatch" errors.

A typical occurence is like:

Get:97 https://repositories.apertis.org/apertis v2019pre/target amd64 libsystemd-dev amd64 240-5co3bb1 [317 kB]
Err:97 https://repositories.apertis.org/apertis v2019pre/target amd64 libsystemd-dev amd64 240-5co3bb1
  Hash Sum mismatch
  Hashes of expected file:
   - SHA256:39654a35430ef132537880d67cd906bc958e1282e5e2d267e0d9ea96198c3649
   - SHA1:3d358b67b624162c4737a619de078cb8ae6091f6 [weak]
   - MD5Sum:c9da96eacf456df58bd564ab587a7a22 [weak]
   - Filesize:317116 [weak]
  Hashes of received file:
   - SHA256:caf4eacc492e6e67651c6d4ace49ee2800c3166e8d630cddd35b87c94042f655
   - SHA1:9690ac45a5282cc04fcbfc6fc3d2ac2e4c6fa375 [weak]
   - MD5Sum:33dcb5800d6e0c3c4d86f0e37c3d134e [weak]
   - Filesize:317116 [weak]
  Last modification reported: Tue, 21 May 2019 14:59:26 +0000

The failures rate goes from hard-to-reproduce to reliably-fails.

Downloading the affected files with wget or curl has not reproduced the issue, and only apt seems affected. The issue has hit jobs on Jenkins as well as pipelines on GitLab, and from time to time people have been able to remporarily reproduce it locally in image builder Docker container.

From the captured network traffic it seems that HTTP pipelining is involved, disabling its usage in APT so far prevented the issue to come up in cases where it was reproducible.