Gives better results in the monitor
Should eval cleanly, as far as -A tarball tells me. Relevant: issue #2999, issue #739
'qgis', one of the few 'qwt' dependees in nixpkgs, fails to build with qwt 6. So I'm not moving the default version away from 5.x. Also, not changing the default allows easy/safe cherry-picking to the stable branch.