Expand environment variables during install (rather than during init)

In conjunction with the new Options.barrier() in buildout,
parts will be able to modify the environment during install/update.

slapos/recipe/cmmi/__init__.py

@@ -140,11 +140,6 @@ class Recipe(object):
         if options.get('configure-command') == 'cygport':
             self.environ.setdefault('CYGCONF_PREFIX', options['prefix'])
 
-        # Extrapolate the environment variables using values from the current
-        # environment.
-        for key in self.environ:
-            self.environ[key] = self.environ[key] % os.environ
-
     def augmented_environment(self):
         """Returns a dictionary containing the current environment variables
         augmented with the part specific overrides.
@@ -274,6 +269,11 @@ class Recipe(object):
                 log.info('This shared package has been installed by other package')
                 return parts
 
+        # Extrapolate the environment variables using values from the current
+        # environment.
+        for key in self.environ:
+            self.environ[key] %= os.environ
+
         make_cmd = self.options.get('make-binary', 'make').strip()
         make_options = ' '.join(self.options.get('make-options', '').split())
         make_targets = ' '.join(self.options.get('make-targets', 'install').split())