Fix run-tempest-26 role for stable/stein

stable/stein uses old Tempest 26.0.0 to run the test
and that is why we use stable/stein upper-constraints
in the testing. But when any jobs with tempest plugins
try to run plugins tests, they fail.

This is because a new role run-tempest-26 (used for testing
with tempest 26.0.0) does not include the stable/stein in the
list of branches for whom we use stable constraints instead of
master constraints. and due to that run-tempest-26 finds constraints
mismatch and recreates the tempest virtual env. On recreation, venv
remove the previously installed tempest pluings and so does not run
their test.

Closes-Bug: #1938227
Change-Id: I00ac642fa602980d14612adc1ce65068d67db8ad
diff --git a/roles/run-tempest-26/tasks/main.yaml b/roles/run-tempest-26/tasks/main.yaml
index b197856..f846006 100644
--- a/roles/run-tempest-26/tasks/main.yaml
+++ b/roles/run-tempest-26/tasks/main.yaml
@@ -29,7 +29,7 @@
   set_fact:
     # TOX_CONSTRAINTS_FILE is new name, UPPER_CONSTRAINTS_FILE is old one, best to set both
     tempest_tox_environment: "{{ tempest_tox_environment | combine({'UPPER_CONSTRAINTS_FILE': stable_constraints_file}) | combine({'TOX_CONSTRAINTS_FILE': stable_constraints_file}) }}"
-  when: target_branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky"]
+  when: target_branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"]
 
 - name: Use Configured upper-constraints for non-master Tempest
   set_fact: