jobs/swh-environment: Ensure swh modules are installed in editable mode

pip install $(./bin/pip-swh-packages --with-testing) has the side effect to get
some swh modules from PyPI because they are declared in some requirements-test
files. This breaks the installation ordering based on the topological sort of
swh module dependencies graph and thus some modules ended up being installed
in non editable mode.

So ensure all swh modules are installed in editable mode first before
installing test dependencies.

Also this requires a recent version of pip as some modules still end up
being intalled in non editable mode without upgrading it.