Use portable shebang in compiler/*.sh

`#!/bin/bash` is non-standard and doesn't work on e.g. NixOS.

Author: GoldsteinE

compiler/build.sh

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

compiler/fetch.sh

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