fixup! build-rina: cache downloads

Same as for commit fcb6a81d,
but for the build of the 'hellorina' SR inside the VM.

software/build-rina/build.cfg

@@ -109,6 +109,9 @@ install =
   extends = /mnt/slapos/software/hellorina/software.cfg
   develop = /mnt/slapos
   download-cache = /mnt/download-cache
+  # Required for recent buildout.
+  extensions -=
+    buildout-versions
   [slapos-cookbook-develop]
   recipe =
   setup =