[python][setup-dunepy.py] Allow to set opts file via env variable like dunecontrol

Timo Kochrequested to merge
fix/python-recognise-optsfile-env-var into master

Dunecontrol allows to specify the opts file via an enviroment variable DUNE_OPTS_FILE so I would expect this option for setup-dunepy.py too. In particular because the two have to match otherwise this may cause compiler errors.

Edited by Timo Koch

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