It's been noted in other issues (JENKINS-35230) that the Git plugin does not export its environment variables when used with a pipeline job. In addition, the plugin does not set user.name and user.email, as it does in traditional jobs, meaning that any attempts to commit to the repository from within a pipeline fail with "tell me who you are".

It would be useful to have the environment variables available, but even without them, the plugin should be able to set the Git config as it performs checkout.