Make the swh-environment jenkins job green and activate notifications

It looks like the effects of caused by https://github.com/pypa/pip/issues/10573 have actually been caught by https://jenkins.softwareheritage.org/job/DENV/job/tests/1654/ but nobody noticed because:

• this job is very flaky https://jenkins.softwareheritage.org/job/DENV/job/tests/buildTimeTrend
• there seem to be no notification (esp. on irc)

Since we do recommend newcomers to use (more or less) what is done in this job as development environment, this job should be stabilized and properly monitored.

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Migrated from T3663 (view on Phabricator)