Refs #25328 Use a portable shebang

tools/apps/update_homebrew/bin/update_homebrew.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library update_homebrew;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 168 matching lines @@
 
     if build.with? 'content-shell'
       content_shell_binary = 'Content Shell.app/Contents/MacOS/Content Shell'
       prefix.install resource('content_shell')
       (bin+"content_shell").write shim_script content_shell_binary
     end
   end
 
   def shim_script target
     <<-EOS.undent
-      #!/bin/bash
+      #!/usr/bin/env bash
       exec "#{prefix}/#{target}" "\$@"
     EOS
   end
 
   def caveats; <<-EOS.undent
     Please note the path to the Dart SDK:
       #{opt_libexec}
 
     --with-dartium:
       To use with IntelliJ, set the Dartium execute home to:
@@ skipping 60 matching lines @@
 
       await runGit(['push']);
     } finally {
       await tempDir.delete(recursive: true);
     }
   }, onError: (error, chain) {
     print(error);
     print(chain.terse);
   });
 }