In addition to update version of slapos.recipe.build, this fixes the
following misuse:
- init phase must only initialize the section, which implies no change
  on the file system
- init/install/update's slapos.recipe.build should not contain buildout
  substitutions (theoritically, there could be wrong escaping,
  e.g. ${buildout:directory} containing quotes)
These 2 points didn't not matter here but it gave a bad example
for anyone learning buildout.