refactor search for python_cmd in PythonPackage and PythonBundle easyblocks

[Flamefire]

Factor out find_python_cmd from PythonPackage and PythonBundle

The logic to find an appropriate python executable is duplicated in the 2 easyblocks leading to slightly diverging implementations.
Combine them into a single function with minor adjustments.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS Flask-3.0.3-GCCcore-13.3.0.eb
• SUCCESS flit-3.9.0-GCCcore-13.2.0.eb
• SUCCESS Mako-1.2.4-GCCcore-13.2.0.eb
• SUCCESS pytest-workflow-2.1.0-GCCcore-13.3.0.eb
• SUCCESS poetry-1.8.3-GCCcore-13.3.0.eb
• SUCCESS pydantic-2.6.4-GCCcore-13.2.0.eb
• SUCCESS scikit-build-0.17.6-GCCcore-13.2.0.eb
• SUCCESS Z3-4.13.0-GCCcore-13.2.0.eb
• SUCCESS lxml-5.3.0-GCCcore-13.3.0.eb
• SUCCESS cryptography-41.0.5-GCCcore-13.2.0.eb
• SUCCESS Cython-3.0.10-GCCcore-13.2.0.eb
• SUCCESS Pillow-10.2.0-GCCcore-13.2.0.eb
• SUCCESS archspec-0.2.2-GCCcore-13.2.0.eb

Build succeeded for 13 out of 13 (13 easyconfigs in total)
n1604 - Linux RHEL 8.9 (Ootpa), x86_64, Intel(R) Xeon(R) Platinum 8470, Python 3.8.17
See https://gist.github.com/Flamefire/3f635faddf4b90c0629d9191241fab61 for a full test report.

[Micket]

lgtm

[Micket]

Test report by @Micket

Overview of tested easyconfigs (in order)

• SUCCESS ReFrame-4.6.2-GCCcore-13.2.0.eb
• SUCCESS ReFrame-4.3.3.eb
• SUCCESS EasyBuild-4.9.3.eb

Build succeeded for 3 out of 3 (3 easyconfigs in total)
vera-skylake-build - Linux Rocky Linux 8.9, x86_64, Intel Xeon Processor (Skylake, IBRS, no TSX), Python 3.6.8
See https://gist.github.com/Micket/6bc7147d739cdc7c4010af829fc05adc for a full test report.

[Micket]

Test report by @Micket

Overview of tested easyconfigs (in order)

• SUCCESS Python-3.11.5-GCCcore-13.2.0.eb
• SUCCESS SciPy-bundle-2024.05-gfbf-2024a.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
vera-skylake-build - Linux Rocky Linux 8.9, x86_64, Intel Xeon Processor (Skylake, IBRS, no TSX), Python 3.6.8
See https://gist.github.com/Micket/35a1b340cd4583f750d899b7e6702f77 for a full test report.