treewide: more portable bash shebangs

treewide: more portable bash shebangs

As in 2a5fcfae024ffc370bb780572279f45a1da3f946
and in 3e67e5c9928f8b1e1c5a63def88d53ed1fed12eb
using /usr/bin/env allows bash to be looked up in PATH
rather than being hard-coded.

As with the previous changes the same arguments apply
- distributions have scripts to rewrite shebangs on installation and
  they know what locations to rely on.
- For tests/compilation we should rather rely on the user to have setup
  there PATH correctly.

In particular this makes testing from git easier on NixOS where do not provide
/bin/bash to improve compose-ability.