Fatal error when exporting submissions and export directory is set to private://

Problem/Motivation

When exporting submissions for a webform, and the export.temp_directory is set to private://, we get:

Uncaught PHP Exception RuntimeException: "Unable to prepare the Webform export directory." at {docroot}/modules/contrib/webform/src/WebformSubmissionExporter.php

Steps to reproduce

Have the export temp directory set to private:// and try and export a form's submissions.

Proposed resolution

The change in https://git.drupalcode.org/project/webform/-/commit/f82f01adc631cf2ac2b3... introduced trimming of the setting before adding /webform at the end. In this case, this results in private:/webform.

On Acquia, this path then fails both is_dir() and is_writable() causing the export to fail.

Remaining tasks

Fix + test.

User interface changes

None.

API changes

None.

Data model changes

None.