[In-app Purchases]Use environment variable to get test site id for IAP#8111

toupper merged 3 commits intotrunkwoocommerce/woocommerce-ios:trunkfrom

issue/8110-site-id-xcode-env-variable-iapwoocommerce/woocommerce-ios:issue/8110-site-id-xcode-env-variable-iapCopy head branch name to clipboard