Site crashes – IMagick issue

After downloading the nightly build this morning, Oct 2, the site has crashed (both public and dashboard) with the msg:

Fatal error: Declaration of WP_Image_Editor_Imagick::supports_mime_type() must be compatible with that of WP_Image_Editor::supports_mime_type() in /home/content/89/4430489/html/mn/wp-includes/class-wp-image-editor-imagick.php on line 410

I need to do a rollback. (I need to figure out how to do a rollback without being able to access the dashboard).