Revert "version up: zc.buildout 2.7.1+slapos006"

This reverts commit de1e9ea0.

2.7.1+slapos006 revealed a bug in buildout bug, breaking resilient KVM.

stack/slapos.cfg

@@ -118,7 +118,7 @@ eggs =
 [versions]
 setuptools = 44.0.0
 # Use SlapOS patched zc.buildout
-zc.buildout = 2.7.1+slapos006
+zc.buildout = 2.7.1+slapos005
 # Use SlapOS patched zc.recipe.egg (zc.recipe.egg 2.x is for Buildout 2)
 zc.recipe.egg = 2.0.3+slapos003
 # Use own version of h.r.download to be able to open .xz and .lz archives