fix: make sure a Rust compiler is available when building

[tonyandrewmeyer]

The COS-lite packages all require packages that have Rust implementations, and the build image doesn't have a Rust compiler, so we need to tell charmcraft to install one.

I've updated the tutorial text as well.

[james-garner-canonical]

I ran into this issue while going through the tutorial. Following lore handed down amongst other tutorial participants, I used a longer list of build-packages to fix it, but testing it just now it turns out that cargo is indeed all you need.