default to __filename when no filename in env
default to __filename when no filename in env

mapbox/carto,pnorman/carto,CartoDB/carto,1ec5/carto,tomhughes/carto,stefanklug/carto,gravitystorm/carto,madeinqc/carto,CartoDB/carto,midnightcomm/carto,clhenrick/carto,whitelynx/carto