Add option to build without rustup (for linux build at least)

[tenzap]

While trying to build an app (which depends on metadata_god) for linux based on flutter I noticed that metadata_god requires to have rustup installed which then will download rust from the internet. However the system can already provide rustc and all the rust ecosystem so that leads to duplicate installation of rustc & co.

Would you add a build option (if that makes sense) to permit building metadata_god without rustup (by using the systems rustc, cargo...)? That would save a lot of bandwidth and disk space especially when running clean builds in clean containers (for packaging systems)

flutter_rust_bridge team has some suggestions in fzyzcjy/flutter_rust_bridge#2900 (comment) on how to do that. For example: https://cjycode.com/flutter_rust_bridge/manual/integrate/template/setup/desktop or fzyzcjy/flutter_rust_bridge#318 (comment)?

[mattpodolak]

Also seeing this issue...