Use /usr/bin/env bash instead of /bin/bash

Summary:
NixOS (like some other distributions) doesn't have bash installed in
/bin.
This would also allow users to use the $PATH to choose a
alternative bash version.

Reviewers: Reviewers, ardumont, vlorentz, olasd

Reviewed By: Reviewers, ardumont, vlorentz, olasd

Subscribers: seirl, olasd, zack, douardda, vlorentz, ardumont

Differential Revision: https://forge.softwareheritage.org/D2793