Speed up CI/CD

[LGro]

Currently, quite some dependencies (flutter, android dev, rust, veilid) are installed and in case of Veilid even compiled. Speed up the CI/CD pipeline(s) by e.g. using pre-built Docker images that carry most of the system dependencies like flutter, android, rust. And as soon as Veilid publishes to pub.dev, get it pre-compiled from there.