Fix problem with variables saving

CHANGELOG

@@ -14,6 +14,7 @@ v7.14.0 (unreleased)
   - Rename type(s) to stage(s)
   - Add missing stage when doing retry
   - Require variable keys to be not-empty and unique
+  - Fix variable saving issue
   
 v7.13.1
   - Fix: user could steal specific runner

app/models/variable.rb

@@ -15,7 +15,7 @@ class Variable < ActiveRecord::Base
   belongs_to :project
   validates_presence_of :key
-  validates_uniqueness_of :key
+  validates_uniqueness_of :key, scope: :project_id
   attr_encrypted :value, mode: :per_attribute_iv_and_salt, key: GitlabCi::Application.secrets.db_key_base
 end