Don't build sources.xml

By setting DISABLE_SOURCES_XML we skip building sources.xml. We don't
want to build it because the generating script is flaky with our
internal manifests.

Change-Id: I88800d5ea16c8c19a023ec9cb007821fb04cf5b9

vendorsetup.sh

@@ -265,3 +265,5 @@ if [[ -d gaia/xulrunner-sdk/.git ]]; then
    export USE_LOCAL_XULRUNNER_SDK=1
 fi
 
+# Don't build sources.xml
+export DISABLE_SOURCES_XML=true