always default create optional environment for deprecated builders

Builder implementation would originally always create an optional
environment instance, which an extended builder's implementation could
reference in their own `__init__` call. However, with the change [1] to
support an `env` argument, there is no longer a guarantee that
`self.env` will exist. This commit changes this to restore the creating
of `self.env` if `env` if not passed in.

[1]: 6ef22d2

Signed-off-by: James Knight <james.d.knight@live.com>

sphinx/builders/__init__.py

@@ -89,10 +89,11 @@ def __init__(self, app: "Sphinx", env: BuildEnvironment = None) -> None:
             self.env: BuildEnvironment = env
             self.env.set_versioning_method(self.versioning_method,
                                            self.versioning_compare)
-        elif env is not Ellipsis:
+        else:
             # ... is passed by SphinxComponentRegistry.create_builder to not show two warnings.
             warnings.warn("The 'env' argument to Builder will be required from Sphinx 7.",
                           RemovedInSphinx70Warning, stacklevel=2)
+            self.env = Optional[BuildEnvironment]  # type: ignore[assignment]
         self.events: EventManager = app.events
         self.config: Config = app.config
         self.tags: Tags = app.tags