Add missing PYTHONPATH variable export to pylint job in CI

The old .gitlab-ci.yaml accidentally worked because pylint
was importing modules internally, probably as a side-effect
of testing other code.

This pylint invocation errors out when PYTHONPATH is not set:
    $ pylint --rcfile $CI_PROJECT_DIR/.pylintrc bin/tests/system/keymgr/
    ************* Module testpolicy
    bin/tests/system/keymgr/ E0401: Unable to import 'isc' (import-error)

And this invocation works even without PYTHONPATH:
    $ pylint --rcfile $CI_PROJECT_DIR/.pylintrc bin/python/ bin/tests/system/keymgr/
79 jobs for !5560 with 2779-wildcard_test_property_based-v9_16 in 45 seconds (queued for 19 seconds)
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