chore: Use the rustup snap to install latest stable rust

[ghislainbourgeois]

Description

This changes the way Rust and Cargo are installed for building the charm. Instead of using the deb packages available for the base, it uses the rustup snap to install the latest stable Rust and Cargo releases.

This will solve building issues for modules like pydantic that now depend on newer releases of Rust to build.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library

[gruyaume]

My feeling here was more or less "let's use what is in the latest Ubuntu". But I understand if we really want the latest pydantic we need this workaround.