Start each script with `#!/usr/bin/env bash`

[sauterp]

On NixOS #!/bin/bash at the start of a script fails, since the bash is somewhere in /nix/store/. We start each script with #!/usr/bin/env bash since that's supported by most distros AFAIK.

[Freed-Wu]

Yes. Fix the shebang is required for https://github.com/NixOS/nixpkgs/issues/243368.