Use /usr/bin/env for bash hashbang instead of /bin/bash
Some OSes don't have bash in /bin, but elsewhere:
- some BSD derivatives
- Nixos
- Guix
- Alpine
This makes those bash scripts works seamlessly in these OSes.
It's extracted from !7177 with only the bash part, which was under debate, so that !7177 can be merged.
For reference, the discussion starts here and continues here