[env] fix pickled doctrees cache (#11939)

CHANGES.rst

@@ -90,6 +90,9 @@ Bugs fixed
   Patch by Bénédikt Tran.
 * #12008: Fix case-sensitive lookup of ``std:label`` names in intersphinx inventory.
   Patch by Michael Goerz.
+* #11474: Fix doctrees caching causing files not be rebuilt in some cases,
+  e.g., when :confval:`numfig` is ``True``.
+  Patch by Bénédikt Tran.
 
 Testing
 -------

sphinx/environment/__init__.py

@@ -265,6 +265,9 @@ class BuildEnvironment:
         """Obtains serializable data for pickling."""
         __dict__ = self.__dict__.copy()
         __dict__.update(app=None, domains={}, events=None)  # clear unpickable attributes
+        # ensure that upon restoring the state, the most recent pickled files
+        # on the disk are used instead of those from a possibly outdated state
+        __dict__.update(_pickled_doctree_cache={})
         return __dict__
 
     def __setstate__(self, state: dict) -> None: