symlink_project.php uses environment variable PROJECT_FILES

To prevent unwanted extra symlinks, the symlink_project.php script now reads a new environment variable named PROJECT_FILES which is created at the start of the Composer job script, before Drupal Core is installed.

If your project has a customized script section in your Composer job, you may want to add
export PROJECT_FILES=$(ls -A)
at the start of the section, to match what the default template now does.

If you only have before_script or after_script in your composer jobs then you do not need to add the extra line.

If you do not define this value, the behavior will fall back into what was done before, which might lead to false errors or warnings being reported on files that are not part of the project.