Fix libpython2.7.a location for static build

ports/python-static/build.sh

 #!/bin/bash
 # Copyright (c) 2014 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 EXECUTABLES=python${NACL_EXEEXT}
 
 # Currently this package only builds on linux.
 # The build relies on certain host binaries and python's configure
 # requires us to set --build= as well as --host=.
@@ skipping 39 matching lines @@
   echo ${PY_LINK_LINE} >> Modules/Setup.local
   # At this point we use the existing environment variables from
   # DefaultConfigureStep to build our destination Python modules
 }
 
 BuildStep() {
   SetupCrossEnvironment
   export CROSS_COMPILE=true
   export MAKEFLAGS="PGEN=${NACL_HOST_PYROOT}/../python-host/build-nacl-host/Parser/pgen"
   DefaultBuildStep
-  ChangeDir ${WORK_DIR}
+  ChangeDir ${BUILD_DIR}
   Banner "Rebuilding libpython2.7.a"
   ${AR} cr libpython2.7.a ${DEST_PYTHON_OBJS}/*.o
   ${RANLIB} libpython2.7.a
   # To avoid rebuilding python.nexe with the new libpython2.7.a and duplicating
   # symbols
   LogExecute touch python${NACL_EXEEXT}
   # The modules get built with SO=so, but they need to be SO=a inside the
   # destination filesystem.
   for fn in `find ${NACL_DEST_PYROOT}/${SITE_PACKAGES} -name "*.so"`
   do
     LogExecute touch ${fn%%so}a
     LogExecute rm -v ${fn}
   done
 }