> While /usr/bin/env is more or less available on all POSIX systems
> /bin/bash might not be. This is particular the case on NixOS and the BSD
> family (/usr/local/bin/bash). Downstream packagers would often rewrite
> those shebangs back automatically as they can rely on absolute paths
> but having portable shebangs in the repository helps to run the code
> without any further modification.
>

The reason almost everyone hardcodes bash to /bin/bash is the potential
environment attack where someone create malicious "bash" and export it in PATH:

https://developer.apple.com/library/archive/documentation/OpenSource/Conceptual/ShellScripting/ShellScriptSecurity/ShellScriptSecurity.html