Merge branch 'feature/dont-regire-pip-II' into 'master'

Martin Nolterequested to merge
cherry-pick-f850031e into releases/2.6

Allow to skip dune_python_package_install when pip is not found II

See merge request !399 (merged)

(cherry picked from commit f850031e)

f40a7649 improve DunePythonInstallPackage to handle missing pip

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