Set SWHPKGDIR on the main sphinx job as well

tox.ini

@@ -11,6 +11,8 @@ extras =
 deps =
   django < 3
   pifpaf
+setenv =
+  SWHPKGDIR = {envsitepackagesdir}/swh
 commands =
   {envpython} -m pifpaf run postgresql -- make -C docs {posargs:html}