Fix relative root in Migrations task (#5266)

* Fix relative root in Migrations task

* Remove slash at start of path

lib/tasks/migrations/migrations.rake

@@ -266,8 +266,8 @@ namespace :migrations do
   def uri(path)
     raise red 'Unable to migrate: No "V3_ENDPOINT" provided, please check your .env file.' unless ENV["V3_ENDPOINT"].present?
 
-    res = URI(ENV["V3_ENDPOINT"])
-    res.path = "/api/v1/migrations/#{path}.json"
+    base_uri = URI(ENV["V3_ENDPOINT"])
+    res = URI::join(base_uri, "api/v1/migrations/#{path}.json")
     res
   end