bayestar-localize-coincs fails if 'file' utility not installed

The ligo.skymap test suite fails when calling bayestar-localize-coincs if the file (/usr/bin/file from the file package on Debian) utility isn't installed. This isn't a standard package in ubuntu:focal containers and similar.

The utility is required to work out the filetype for some files, is is possible to just read the magic characters from the file headers instead?