Fix shebang in bash build scripts

Looking for BASH in the environment instead of invoking it directly
enables attacks where malware could escalate privileges by pointing BASH
to itself and tricking the user into entering a superuser password when
a user invokes our script, a familiar request the user may be expecting.

scripts/install_deps.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/bash
 
 apt-get update
 apt-get update --fix-missing

scripts/pinned_build.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/bash
 set -eo pipefail
 
 echo "Leap Pinned Build"