jobs/swh-environment: Mitigate backtracking issue with new pip resolver

pip 20.3 just got released and set 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, workaround the issue by splitting
out the pip command into multiple ones (one by swh package).