Builder.build: save env also with only new documents

Saving/pickling the environment is necessary for intersphinx to have its
cache saved.

Currently a new build would fetch the inventories, but the cache would
not get saved.

This appears to break no existing test, but should probably get a new
one then?

sphinx/builders/__init__.py

@@ -372,14 +372,14 @@ def build(self, docnames, summary=None, method='update'):
         else:
             logger.info(__('none found'))
 
-        if updated_docnames:
-            # save the environment
-            from sphinx.application import ENV_PICKLE_FILENAME
-            logger.info(bold(__('pickling environment... ')), nonl=True)
-            with open(path.join(self.doctreedir, ENV_PICKLE_FILENAME), 'wb') as f:
-                pickle.dump(self.env, f, pickle.HIGHEST_PROTOCOL)
-            logger.info(__('done'))
+        # save the environment
+        from sphinx.application import ENV_PICKLE_FILENAME
+        logger.info(bold(__('pickling environment... ')), nonl=True)
+        with open(path.join(self.doctreedir, ENV_PICKLE_FILENAME), 'wb') as f:
+            pickle.dump(self.env, f, pickle.HIGHEST_PROTOCOL)
+        logger.info(__('done'))
 
+        if updated_docnames:
             # global actions
             self.app.phase = BuildPhase.CONSISTENCY_CHECK
             logger.info(bold(__('checking consistency... ')), nonl=True)