phosphor-ipmi-sensor: update WORKDIR usage

The latest yocto release(styhead,5.1) has made a significant change[1]
to the use of the WORKDIR parameter.

Update the recipe to handle this.

Change-Id: I8898d3ab8dfe60d1d8a8a708650327f548347bcc
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-sensor-inventory-mrw-native.bb b/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-sensor-inventory-mrw-native.bb
index 139c9b9..ed77f31 100644
--- a/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-sensor-inventory-mrw-native.bb
+++ b/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-sensor-inventory-mrw-native.bb
@@ -37,7 +37,7 @@
     import subprocess
 
     # TODO: Perform the merge in a temporary directory?
-    workdir = d.getVar('WORKDIR', True)
+    workdir = d.getVar('UNPACKDIR', True)
     nativedir = d.getVar('STAGING_DIR_NATIVE', True)
     sensoryamldir = d.getVar('sensor_yamldir', True)
     cmd = []