This commit changes the shebang in all shell scripts from absolute
paths (such as `/bin/bash` or `/usr/bin/python3`) to the `/usr/bin/env`
binary with the accompanying interpreter as it's argument.
The reason for this are as follows:
- NixOS cannot work with absolute paths
- BSD systems install their packages in /usr/local/bin
A build script reaching into your .git/hooks/ and modifying them
nonconsensually was a bit of a horrifying concept, and also made it hard
to build arti with the feature disabled. Remove this crate, and replace
it with manual instructions on how to install the hooks in
CONTRIBUTING.md.