restore original $PATH and $PYTHONPATH values after installing first EasyBuild module

[boegel]

When running the install script to build the software from scratch (for a new type of CPU for example), the first non-EasyBuild installation fails hard because it's accidentally picking up stuff from the EasyBuild v4.7.0 installation that was installed with pip install:

== 2023-01-24 12:47:20,473 build_log.py:169 ERROR EasyBuild crashed with an error (at easybuild/base/exceptions.py:124 in __init__):
Failed to process easyconfig /cvmfs/pilot.eessi-hpc.org/versions/2021.12/software/linux/aarch64/altra/software/EasyBuild/4.5.0/easybuild/easyconfigs/j/Java/Java-11.eb: import_available_modules:
Failed to import easybuild.toolchains.gmpit: cannot import name 'LooseVersion' from 'easybuild.tools' (/cvmfs/pilot.eessi-hpc.org/versions/2021.12/software/linux/aarch64/altra/software/EasyBuild/4.5.0/lib/python3.9/site-packages/easybuild/tools/__init__.py) (at easybuild/framework/easyconfig/easyconfig.py:2071 in process_easyconfig)

The import_available_modules calls is picking up on easybuild/toolchains/*.py in the EasyBuild v4.7.0 installation that was installed in a temporary directory, because that location is still listed in $PYTHONPATH, but those files use:

from easybuild.tools import LooseVersion

which is not compatible with EasyBuild v4.5.0 (cfr. easybuilders/easybuild-framework#3794)