deps: add rust build dependencies to charm

[lengau]

Issue

493

Solution

Adds rustc and cargo as build packages in the

Context

The jsonschema update broke chramcraft's spread tests

Testing Instructions

Build the charm :-)

Release Notes

Add build packages so newer jsonschema versions work.

[lengau]

That's a good question @sed-i . I sent this because of the general preference for building everything from source for production charms, but getting the binary instead is indeed much easier 😀

[sed-i]

CI passed, but it is not reflected on the PR page here. Merging.