Replace shebang in build.sh with ShellCheck directive

This script is meant to be sourced by the OSS-Fuzz file of the same
name, rather than executed directly. The shebang may lead to the
incorrect assumption that the script is meant for direct execution.
Replacing it with this directive instructs ShellCheck to treat
the script as a Bash script, regardless of how it is executed.

Based @EliahKagan's suggestion and feedback on:
#1901

fuzzing/oss-fuzz-scripts/build.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+# shellcheck shell=bash
 
 set -euo pipefail