force a clean before the build to see if this makes bamboo

fail less often.

.nant/bamboo.build

@@ -7,6 +7,7 @@
   <target name="cibuild">
     <property name="projectdir" value="opensim-0.5.9" />
     <exec program="mono" commandline="bin/Prebuild.exe /target nant" workingdir="../" />
+    <nant buildfile="../OpenSim.build" target="clean" />
     <nant buildfile="../OpenSim.build" target="build" />
 
     <delete dir="../${projectdir}" />