EbuildFetcher: fix fetch to use manifest

If we don't call doebuild_environment() then the 'O' variable is
not set and fetch() doesn't use the manifest.
diff --git a/pym/_emerge/EbuildFetcher.py b/pym/_emerge/EbuildFetcher.py
index 1abe846..141dc69 100644
--- a/pym/_emerge/EbuildFetcher.py
+++ b/pym/_emerge/EbuildFetcher.py
@@ -46,6 +46,8 @@
 
 		settings = self.config_pool.allocate()
 		settings.setcpv(self.pkg)
+		portage.doebuild_environment(ebuild_path, 'fetch',
+			settings=settings, db=portdb)
 
 		if self.prefetch and \
 			self._prefetch_size_ok(uri_map, settings, ebuild_path):