Ruby scripts launched from TM are fed directly to the ruby interpreter so unless IT interprets the shebang (and I don't think so), putting one at the start will be ignored.
As for using env... well, in the Run Ruby command, ruby is started like this: ruby ..and given that shell commands started from TM enjoy a full login- shell setup, your PATH should also be correct, leading to the 'usual' ruby being invoked.
-- Sune.