Fix python_exe_dirs values on jammy and focal jobs

In jobs run on ubuntu noble, python executable has to be found on
virtual environments because installing packages with pip at system
level is not possible.
In jobs run on both jammy and focal, pip can install packages at system
level, so those venvs should not be used.

Change-Id: Ie7d09f26c3e611567d497148a78712f0efee8c5b

zuul.d/devstack-tobiko.yaml

@@ -155,7 +155,13 @@
     roles:
       - zuul: x/tobiko
     timeout: 10800
-    vars: *job_vars
+    vars:
+      <<: *job_vars
+      python_exe_dirs:
+        - ~/.local/bin
+        - ~/bin
+        - /usr/local/bin
+        - /usr/bin
     pre-run: playbooks/devstack/pre.yaml
     run: playbooks/devstack/run.yaml
     post-run: playbooks/devstack/post.yaml
@@ -178,7 +184,13 @@
     roles:
       - zuul: x/tobiko
     timeout: 10800
-    vars: *job_vars
+    vars:
+      <<: *job_vars
+      python_exe_dirs:
+        - ~/.local/bin
+        - ~/bin
+        - /usr/local/bin
+        - /usr/bin
     pre-run: playbooks/devstack/pre.yaml
     run: playbooks/devstack/run.yaml
     post-run: playbooks/devstack/post.yaml