Make the build system's `cleanall` option really clean everything.

At the moment it doesn't clean the cached wiki build.

vagrant/provision/assets/build-tails

@@ -99,6 +99,7 @@ install -m 0755 -d "$ARTIFACTS_DIR"
 
 if [ "$TAILS_CLEAN_BUILD" ]; then
 	as_root_do lb clean --all
+	git clean -fdx
 fi
 
 if [ -z "$JENKINS_URL" ]; then