mgr/dashboard: Fix tox warning about PYTHONPATH

mgr/dashboard: Fix tox warning about PYTHONPATH

Get rid of the warning "WARNING: Discarding $PYTHONPATH from environment, to override specify PYTHONPATH in 'passenv' in your configuration." by declaring "passenv=PYTHONPATH" according to the tox documentation.

> PYTHONPATH will be passed down if explicitly defined. If PYTHONPATH exists in the host environment but is not declared in passenv a warning will be emitted.

See https://tox.readthedocs.io/en/latest/config.html?highlight=passenv#conf-passenv.

Signed-off-by: Volker Theile <vtheile@suse.com>