🐛 Register {temp_dir} as a valid substitution

This change recovers a missing `temp_dir` substitution.
The docs for it were added in tox-dev#1032 but it never actually worked.

Fixes tox-dev#1608

Author: webknjaz

src/tox/config/__init__.py

@@ -1102,8 +1102,9 @@ def line_of_default_to_zero(section, name=None):
 
         reader.addsubstitutions(distdir=config.distdir)
         config.distshare = reader.getpath("distshare", dist_share_default)
-        config.temp_dir = reader.getpath("temp_dir", "{toxworkdir}/.tmp")
         reader.addsubstitutions(distshare=config.distshare)
+        config.temp_dir = reader.getpath("temp_dir", "{toxworkdir}/.tmp")
+        reader.addsubstitutions(temp_dir=config.temp_dir)
         config.sdistsrc = reader.getpath("sdistsrc", None)
         config.setupdir = reader.getpath("setupdir", "{toxinidir}")
         config.logdir = config.toxworkdir.join("log")