Use BZR_PLUGINS_AT and stop mucking with BZR_PLUGIN_PATH.
* bzrlib/tests/test_plugins.py:
(TestLoadPluginAt.test_import): Update test.
* bzrlib/tests/__init__.py:
(TestCase._cleanEnvironment): Add BZR_PLUGINS_AT.
* bzrlib/plugin.py:
(set_plugins_path): Use a dedicated BZR_PLUGINS_AT env var .