[BUG] babel runs bash instead of zsh when sessions are used

[Rudi C]

#+BEGIN_SRC zsh :session z1
echo $HOME $0
#+END_SRC

#+RESULTS:
: /Users/evar /bin/bash

As compared to:

#+BEGIN_SRC zsh
echo $HOME $0
#+END_SRC

#+RESULTS:
: /Users/evar zsh