Setup proper zuul_work_dir

This avoids other projects running our jobs from installing their
bindep.txt files.

Change-Id: Iddc755f16f51853d953e74df5d8848f24e3d9f07
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

.zuul.d/jobs.yaml

@@ -33,6 +33,8 @@
       - zuul/nodepool
       - zuul/zuul
       - zuul/zuul-registry
+    vars:
+      zuul_work_dir: "{{ zuul.projects['opendev.org/windmill/windmill'].src_dir }}"
     timeout: 3600
 
 # Testing for install_method: pip