Add a missing escape to build.yml

We want to actually write '$@', but without escape it gets replaced
with the arguments used to run the script itself.

.github/workflows/build.yml

@@ -110,7 +110,7 @@ jobs:
           preflight do
             File.write wrapper_script, <<~EOS
               #!/bin/sh
-              "#{staged_path}/openfortivpn-webview.app/Contents/MacOS/openfortivpn-webview" "$@"
+              "#{staged_path}/openfortivpn-webview.app/Contents/MacOS/openfortivpn-webview" "\$@"
             EOS
           end