server: build: use a more portable shebang for bash

This patch should increase the portability of bash scripts across various *nix systems
such as BSD-like distributions.

Signed-off-by: Luís Ferreira <contact@lsferreira.net>

server/build_without_gradle.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # This script generates the scrcpy binary "manually" (without gradle).
 #