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

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

changed milestone to %DUNE 2.6.0

enabled an automatic merge when the pipeline for ef6adf49 succeeds

mentioned in commit 70725acb

merged