Unset CDPATH if it is set.

Apparently on some (probably buggy) platforms, when CDPATH is set, the
'cd' command will always print text to stdout.  This caused fail 122
and 123 in shelltest.od.

CDPATH is an interactive-mode option, so let's clear it when running
scripts.

redo/builder.py

@@ -309,6 +309,8 @@ class _BuildJob(object):
         # now.
         assert state.is_flushed()
         newp = os.path.realpath(dodir)
+        # CDPATH apparently caused unexpected 'cd' output on some platforms.
+        os.unsetenv('CDPATH')
         os.environ['REDO_PWD'] = state.relpath(newp, env.v.STARTDIR)
         os.environ['REDO_TARGET'] = basename + ext
         os.environ['REDO_DEPTH'] = env.v.DEPTH + '  '