changed shebangs to be compatible to non-FHS distros

[AtilaSaraiva]

Hello, thank you for the amazing script. I was here trying to use the script on NixOS and noticed that there is a mix of shebangs being used in the script. Most of the shell files use #!/bin/bash, and some others use #!/usr/bin/env bash. However, the former is not compatible with non-FHS compliant linux distros, hence I'm here submitting this PR to make every script use a more compatible approach.

Thank you all in advance.

[Flightkick]

It is a good practice to use the env instead of /bin/bash. There's a slight inconsistency in the changeset however. Some files incorrectly refer to /usr/env/bin which should be /usr/bin/env instead.