Let's teach gpython and pymain about -E (ignore $PYTHON* environment
variables) because new buildout runs python -E inside. Xavier reports:

    Since slapos was upgraded zc.buildout 3.0.1+slapos004, tests for
    slapos.rebootstrap and slapos.recipe.template fail because buildout now
    installs in develop with pip install --editable instead of python
    setup.py develop and in the process pip runs python -E, e.g.
    https://erp5js.nexedi.net/#/test_result_module/20240912-837A12F7/10

For the implementation use the same approach to reexecute underlying
interpreter with given low-level option as we already did for -O in
8564dfdd (gpython: Implement -O).

/reported-and-tested-by @xavier_thompson
/reviewed-by @jerome
/reviewed-on nexedi/pygolang!30