CI: disable pip cache
Description
The CI shows sporadic failures
$ venv/bin/python3 -m pip install --quiet junitparser
WARNING: Skipping page https://pypi.org/simple/junitparser/ because the GET request got Content-Type: .The only supported Content-Type is text/html
ERROR: Could not find a version that satisfies the requirement junitparser
ERROR: No matching distribution found for junitparser
This seems to be a bug in pip, potentially related to the cache: https://github.com/pypa/pip/issues/11340
The issue reports that disabling the cache would solve the problem. If it does not we can provide junitparser on the machines instead of installing it in the CI jobs.