encode default path for channel list
It would be very convenient if the ligo_channel_completion binary would recognize default paths to the channel list database if the CHAN_LIST environment variable is not defined. That would make it much easier for administrators to just drop the channel list into a known location and have it Just Work. Maybe the ordering should be something like:
CHAN_LIST
~/.cache/ligo-channel-list/chan-list.txt
/var/cache/ligo-channel-completion/chan-list.txt