phpunit-junit builder missed full path to phpunit

phpunit is deployed under /srv/deployment/integration/phpunit using
integration/phpunit.git.  Reference the full path instead of simply
'phpunit'.

That fix the job wikimedia-wikimania-scholarships-phpunit which was
failling when run on lanthanum (there is no phpunit in PATH on that
slave).

bug: 57572
Change-Id: I0db0fef4a1b34ef0d51c0d8e51c0703156d8861a