build: cmake doesn't install python link

[grondo]

When flux(1) prepends to PYTHONPATH it points to ${libdir}/flux/python3.x instead of the real Python path at ${libdir}/python3.x/site-packages. Symlinks are then added here as necessary, e.g.:

/usr/lib64/flux/python3.6/flux -> /usr/lib64/python3.6/site-packages/flux
/usr/lib64/flux/python3.6/_flux -> /usr/lib64/python3.6/site-packages/_flux

This keeps flux(1) from inadvertently prepending the system Python paths to a user's PYTHONPATH while still retaining the ability to push the correct bindings to the front of that path.

The fluxion link here was dropped with the conversion to cmake and needs to somehow be "installed" with make install, or else those python modules can't be loaded.