taanab osmesa builds working again

Recently taanab ran out of disk space, causing builds to fail. I presume to save space the superbuild that provided osmesa was removed. This caused the osmesa builds to fail, obviously.

I restored a stripped down superbuild with only the ENABLE_osmesa option on. This provides the OSmesa library under /home/kitware/code/pvsb/build-mesa/install, fixing the osmesa build issue.

There were a few tests consistently failing on this dashboard. On closer inspection, it was due to the fact that this SB built a Python (which is needed for mesa build). ParaView builds ended up finding this Python instead on the one of the system causing the issues. I’m rebuilding the superbuild with USE_SYSTEM_python2=ON. Hopefully that gets this dashboard back to green.