Only the owner of the directory, root, can change its permissions, allowing or not other users to use this directory (note that root, as the superuser, can anyway change permissions of quite anything, owner or not).
If user jenkins is in the wheel group, this might mean, depending on the exact OS, that user jenkins is allowed to use
sudo (eg: RHEL) to temporarily become root. So this, perhaps needing jenkins' password, should work:
sudo chmod -R a+rwx tmp/
or else (needing root's password):
su -c 'chmod -R a+rwx tmp/'
Being in the group wheel, if treated in a special way by your OS, doesn't give any direct permission. It grants the right to execute things as root (or other users) using various mechanisms intended for this effect (like
sudo above), not directly.