Store a copy of the slave's path module on Build.

This is saves navigating to through the slavebuilder
and slave each time this is needed.

master/buildbot/process/build.py

@@ -184,13 +184,15 @@ def setupProperties(self):
     def setupSlaveBuilder(self, slavebuilder):
         self.slavebuilder = slavebuilder
 
+        self.path_module = slavebuilder.slave.path_module
+
         # navigate our way back to the L{buildbot.buildslave.BuildSlave}
         # object that came from the config, and get its properties
         buildslave_properties = slavebuilder.slave.properties
         self.getProperties().updateFromProperties(buildslave_properties)
         if slavebuilder.slave.slave_basedir:
             self.setProperty("workdir",
-                    slavebuilder.slave.path_module.join(
+                    self.path_module.join(
                         slavebuilder.slave.slave_basedir,
                         self.builder.config.slavebuilddir),
                     "slave")