environment vars need to be exported in order to make duplicity work when used inside a script

this fixes authentication error:
```
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1251, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1244, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1130, in main
    action = commandline.ProcessCommandLine(sys.argv[1:])
  File "/usr/lib/python2.6/dist-packages/duplicity/commandline.py", line 908, in ProcessCommandLine
    backup, local_pathname = set_backend(args[0], args[1])
  File "/usr/lib/python2.6/dist-packages/duplicity/commandline.py", line 803, in set_backend
    globals.backend = backend.get_backend(bend)
  File "/usr/lib/python2.6/dist-packages/duplicity/backend.py", line 153, in get_backend
    return _backends[pu.scheme](pu)
  File "/usr/lib/python2.6/dist-packages/duplicity/backends/botobackend.py", line 105, in __init__
    self.conn = S3Connection()
  File "/usr/lib/python2.6/dist-packages/boto/s3/connection.py", line 167, in __init__
    suppress_consec_slashes=suppress_consec_slashes)
  File "/usr/lib/python2.6/dist-packages/boto/connection.py", line 525, in __init__
    host, config, self.provider, self._required_auth_capability())
  File "/usr/lib/python2.6/dist-packages/boto/auth.py", line 440, in get_auth_handler
    'Check your credentials' % (len(names), str(names)))
NoAuthHandlerFound: No handler was ready to authenticate. 1 handlers were checked. ['HmacAuthV1Handler'] Check your credentials
```