Default bubblewrap to work_root

Default chdir to jobdir.work_dir for bubblewrap and start
running our commands from there.

Change-Id: Ied3d13bc4257c669a6bbb30750f154dcf5e3b970
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/zuul/driver/bubblewrap/__init__.py b/zuul/driver/bubblewrap/__init__.py
index c93e912..083a384 100644
--- a/zuul/driver/bubblewrap/__init__.py
+++ b/zuul/driver/bubblewrap/__init__.py
@@ -90,7 +90,7 @@
         '--bind', '{work_dir}', '{work_dir}',
         '--dev', '/dev',
         '--dir', '{user_home}',
-        '--chdir', '/',
+        '--chdir', '{work_dir}',
         '--unshare-all',
         '--share-net',
         '--uid', '{uid}',