set dashcard js_env variables before branching on ff
set dashcard js_env variables before branching on ff

flag=react_discussions_post

Change-Id: I698b6817a4b8a04dc91f66e5df8ff11ec2e47965
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/273200
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
QA-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Product-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms