fixup! build-rina: cache downloads

Error: Buildout now includes 'buildout-versions' (and part of the older 'buildout.dumppickedversions').

software/build-rina/software.cfg

@@ -8,6 +8,10 @@ parts =
   template
 download-cache = download-cache
 
+# Required for recent buildout.
+extensions -=
+  buildout-versions
+
 [template]
 recipe = slapos.recipe.template:jinja2
 # XXX: "template.cfg" is hardcoded in instanciation recipe