[infra] Escape $ in new homebrew script string

Change-Id: I9f4f675372589929c117e73cfc445e7b13c30f0f
Reviewed-on: https://dart-review.googlesource.com/59740
Reviewed-by: William Hesse <whesse@google.com>

tools/apps/update_homebrew/lib/update_homebrew.dart

@@ -234,7 +234,7 @@ class DartAT2 < Formula
   def shim_script(target)
     <<~EOS
       #!/usr/bin/env bash
-      exec "#{prefix}/#{target}" "$@"
+      exec "#{prefix}/#{target}" "\$@"
     EOS
   end
 
@@ -258,7 +258,6 @@ class DartAT2 < Formula
 end
 ''';
 
-
 Future runGit(List<String> args, String repository,
     Map<String, String> gitEnvironment) async {
   print("git ${args.join(' ')}");