Catkin plugin: empty segment added to PYTHONPATH

[Kyle Fazzari]

The Catkin plugin puts something along the following lines into the environment:

    export PYTHONPATH=foo:$PYTHONPATH

Which apparently is a bad thing, because if $PYTHONPATH is empty, that puts an empty segment on the PYTHONPATH which Python interprets as `.`, thus adding the current working directory to it. There are some modules that walk the PYTHONPATH (e.g. pkg_resources), and the current working directory is often inaccessible to the snap in confinement. No idea why Python works this way, but then again, I feel that way a lot toward Python. This needs to be fixed in the Catkin plugin to only append the current PYTHONPATH if it's actually set.

[Kyle Fazzari (kyrofa)]

Fix is here: https://github.com/snapcore/snapcraft/pull/1522