Revert "Builder.build: save env also with only new documents"

This reverts commit 20f625b4d3, which
introduced a regression in dependency tracking for build systems
invoking sphinx-build (described in issue #5501.)

The correct solution is to properly track when the pickle file needs to
be written;  unfortunately I don't know enough about sphinx to implement
this.  Also, I'm not sure the intersphinx cache belongs into this at
all.

sphinx/builders/__init__.py

@@ -369,6 +369,7 @@ class Builder(object):
         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)
@@ -376,7 +377,6 @@ class Builder(object):
                 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)