Fix tox in Azure CI

Fixes: https://pagure.io/freeipa/issue/9347

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Francisco Trivino <ftrivino@redhat.com>

tox.ini

@@ -5,6 +5,7 @@ skip_missing_interpreters=true
 skipsdist=true
 
 [testenv]
+allowlist_externals={toxinidir}/.tox-install.sh
 # always re-create virtual env. A special install helper is used to configure,
 # build and install packages.
 recreate=True