fix: use env to find bash interpreter

Using `/bin/bash` may cause issues, where some Linux distribution do not
place the bash binary under `/bin` eor adhere to Filesystem Hierarchy
Standard (FHS), e.g., NixOS.

Author: arminveres

packages/gitbook/scripts/clean.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -o errexit
 set -o pipefail

packages/gitbook/scripts/generate.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -o errexit
 set -o pipefail