CMT based test jobs crashing

After !274 (merged) test jobs for CMT builds fail with:

Traceback (most recent call last):
  File "/workspace/venv/slc5/bin/lbn-test", line 14, in <module>
    sys.exit(Script().run())
  File "/workspace/venv/slc5/lib/python2.7/site-packages/LbCommon/Script.py", line 107, in run
    rc = self.main()
  File "/workspace/venv/slc5/lib/python2.7/site-packages/LbNightlyTools/Scripts/Test.py", line 207, in main
    call([CTEST_CONVERTER], cwd=self._buildDir(proj, 'build'))
  File "/cvmfs/lhcb.cern.ch/lib/lcg/external/Python/2.7.3/x86_64-slc5-gcc46-opt/lib/python2.7/subprocess.py", line 493, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/cvmfs/lhcb.cern.ch/lib/lcg/external/Python/2.7.3/x86_64-slc5-gcc46-opt/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/cvmfs/lhcb.cern.ch/lib/lcg/external/Python/2.7.3/x86_64-slc5-gcc46-opt/lib/python2.7/subprocess.py", line 1249, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory: '/workspace/build/LHCb/build'