autoPatchelfHook: Make easier to run autoPatchelf

The autoPatchelf main function which is run against all of the outputs
was pretty much tailored towards this specific setup-hook and was
relying on $prefix to be set globally.

So if you wanted to run autoPatchelf manually - let's say during
buildPhase - you would have needed to run it like this:

  prefix=/some/directory autoPatchelf

This is now more intuitive and all you need to do is run the following:

  autoPatchelf /some/directory

Signed-off-by: aszlig <aszlig@nix.build>