follow-up to !1719 "Use full path in sub files for default lalapps_MakeSFTDAG options"

Hi @evan-goetz @karl-wette sorry I didn't see this in time, but I don't think !1719 (merged) isn't quite the right solution. Don't these hardcoded /usr/bin paths mean that if I run lalapps_MakeSFTDAG from a conda env or venv, I will get the system-installed lalapps_MakeSFTs instead of the one from the environment...? There's already the possibility of course to set the path manually via --makesfts-path or $MAKESFTS_PATH. But as a fall-back if the user hasn't done either, isn't there some default path environment variable set by lalsuite that we can use, or maybe we could resort to asking which?