[Forwardport] Transport variable can not be altered in email_invoice_set_template_vars_before Event

[gelanivishal]

Original Pull Request

#15040

Fix missing commits from #10210 Transport variable can not be altered in email_invoice_set_template_vars_before Event

Description

Original commit in #10210 missed updates to the /app/code/Magento/Sales/Model/Order/Email/Sender/OrderSender.php file

Fixed Issues (if relevant)

1. Transport variable can not be altered in email_invoice_set_template_vars_before Event #10210

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @gelanivishal. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me {$VERSION} instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[sidolov]

Hi @gelanivishal , this issue was ported to 2.3 release line in Pull Request: #15039 and was reverted due to Breaking Changes in #16600.
I'm closing your PR as not relevant.
Thank you for the contribution!