Consider using system-wide askpass and app-placeholder instances

Following https://github.com/helloSystem/ISO/commit/d12eb2f1af324a3c1a57889cc30bea9639503a62 we can consider using system-wide instances instead of bundling them privately inside each placeholder `.app` bundle:

```
/usr/local/bin/askpass
/usr/local/libexec/app-placeholder
```

Doing so would increase maintainability (less duplication) at the expense of making the placeholder `.app` bundles run only on helloSystem (or any system that ships the above).