CI: limit upper version of setuptools<50 for Jenkins build

[sphuber]

Fixes #4342

Builds on Jenkins started failing after setuptools==0.50.0 was
released on August 30, 2020. This new version gets automatically
installed when using a pyproject.toml for the build system, but it
causes the installation of the aiida-core package through pip to
fail with the exception:

ModuleNotFoundError: No module named 'setuptools._distutils'

Temporarily limiting the version of setuptools in the pyproject.toml
works around the problem for the time being.

The build is updated to also update the version of pip before
installing the package.

[sphuber]

Relevant issue on setuptools github

[codecov]

Codecov Report

Merging #4343 into develop will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4343      +/-   ##
===========================================
+ Coverage    79.14%   79.16%   +0.03%     
===========================================
  Files          468      468              
  Lines        34616    34616              
===========================================
+ Hits         27395    27402       +7     
+ Misses        7221     7214       -7     

Flag	Coverage Δ
#django	72.77% <ø> (+0.01%)	⬆️
#sqlalchemy	71.97% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiida/transports/plugins/local.py	81.54% <0.00%> (+0.26%)	⬆️
aiida/engine/daemon/runner.py	82.76% <0.00%> (+3.45%)	⬆️
aiida/orm/utils/log.py	93.34% <0.00%> (+16.67%)	⬆️

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[sphuber]

Put this PR on hold to see if a fix downstream becomes available in a day or two that makes the fix obsolete. Otherwise we will merge

[sphuber]

One user in the above mentioned thread, actually argues it is a bug in pip and not setuptools. They describe their that they suspect that using a virtualenv with --system-site-packages breaks pip's build isolation. I tried looking into the Jenkins configuration file what type of environment is used, but it looks to me like we are not even using a virtual env at all? @giovannipizzi is that true? If that is the case, then there might be another reason for the problem we have been seeing. Anyway, it doesn't seem like there will be a fix downstream, so I think we should just merge this.