shebang: changed scripts shebang to better support systems like bsd and nixos

[lromor]

The assumption that bash is always located at /bin/bash does not always hold true.
In POSIX systems, it is recommended to avoid relying on common paths. Instead, one should either:

• Hardcode the full absolute path to the executable, or
• Locate the command in the PATH.

A more portable approach is to use #!/usr/bin/env bash, which resolves binaries through the PATH.

This patch updates the default shebang string in this repository to improve portability on systems like NixOS or BSD.

[hzeller]

Nice! This will unbreak systems that behave Posix-correctly and don't have bash in /bin.