Yeah - so the translator can only in practice translate for the current platform.  If the pants.ini or python_binary asks for a non-native platform, without this fix, a SourceLink will happily build for the native platform and return success.  We ask for linux - it hands us back mac.  This in fact accounts for many surprise build on mac, deploy to linux runtime failures at Twitter today.  I tried 3 variants of the fix - the other 2 were in Obtainer and in *Link - I landed here.