Issue #6485: Fix Script-fu brush creation scripts
This patch attempts to resolve #6485 (closed) for 2.10 (it's not an issue in 2.99).
These scripts call "file-gbr-save" to save the created brush. Currently they pass in an empty string for the "raw-uri" parameter, which is not permitted - so they throw an error and don't save the brush.
This can be resolved by replacing the "" with the filename variable. This is apparently how it was solved for the paste-as-brush script: https://gitlab.gnome.org/GNOME/gimp/-/blob/gimp-2-10/plug-ins/script-fu/scripts/paste-as-brush.scm#L47