Temporarily pin Yarn to fix CI

Author: gaearon

tasks/e2e-simple.sh

@@ -90,7 +90,7 @@ fi
 if [ "$USE_YARN" = "yes" ]
 then
   # Install Yarn so that the test can use it to install packages.
-  npm install -g yarn
+  npm install -g yarn@0.22 # FIXME: this pin is temporary to work around a Yarn bug on CI
   yarn cache clean
 fi