jobs/swh-environment: Mitigate backtracking issue with new pip resolver
pip 20.3 just got released and sets its new resolver as default.
However it does not seem stable enough yet as the pip command to install swh dependencies now ends up executing a really long package backtracking process that makes the swh-environment Jenkins job timeout.
Until this got sorted out by pip developers (https://github.com/pypa/pip/issues/9187), workaround the issue by splitting out the pip command into multiple ones (one by swh package).
Migrated from D4648 (view on Phabricator)