Fix swh-environment's jenkins build

It's been red for a while now.

The gist of what's running for the build is [1]:

#!/bin/bash

mr -j 4 -t update
python3 -m venv .venv
. .venv/bin/activate
pip install wheel
pip install pifpaf pytest
pifpaf run postgresql echo "pifpaf OK"

pip install $(bin/pip-swh-packages --with-testing)
pifpaf run postgresql sh -- -c 'for d in swh-*; do (echo Testing in $d; cd $d; pytest); done' [2]

• [1] https://jenkins.softwareheritage.org/job/swh-environment/configure

• [2] is breaking either for missing dependencies (pip) or system one

We need to take the time to analyze further and improve the current way of doing it.

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Migrated from T1440 (view on Phabricator)