tools/toolbox: Use /usr/bin/env for bash
/bin/bash is non-standard, and in particular isn't present on NixOS. The standard way to handle scripting languages other than posix shell is via /usr/bin/env, so this change switches to using it.
/bin/bash is non-standard, and in particular isn't present on NixOS. The standard way to handle scripting languages other than posix shell is via /usr/bin/env, so this change switches to using it.