[skip ci][FIX] Revising antsBrainExtraction dual workflow
[skip ci][FIX] Revising antsBrainExtraction dual workflow

I'm checking both workflows command-by-command, so I'll be gathering here the changes necessary to make them fully dual.
oesteban/niworkflows,poldracklab/niworkflows,poldracklab/niworkflows,oesteban/niworkflows,oesteban/niworkflows