Add zuul.executor.work_root to inventory

It is helpful to expose the work_root into the inventory. Today, we
would need to use a relative path on log_root or src_root. Having the
absolute path is better to read for debugging.

Change-Id: I86ff459d283eaf348821c2f11c1f8575598f088d
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index b6a8564..f533f54 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -322,6 +322,8 @@
 **zuul.executor.log_root**
   The path to the logs directory.
 
+**zuul.executor.work_root**
+  The path to the working directory.
 
 .. _user_sitewide_variables: