README: Fix Build Dependencies for Ubuntu Distros

[bradloh]

Description

I worked through dependency issues for a source build on Ubuntu 25.10, then re-tested the fixes in a 24.04 build container. Contributing the lessons learned upstream.

Changed

• Command formatting: for better readability and better diffs/tracking
• rustc and cargo are now advised to be installed by rustup. This is because the rustc Ubuntu package is too old on both 24.04 and 25.10.
• just is now advised to be installed by cargo. This is because just is too old on 24.04
• cargo is now listed as required instead of optional (partly due to the above change).

Added

• 3 missing libraries that caused fatal build failures. It's worth noting that additional packages are available on apt that install rust bindings for these libraries (e.g. apt install rustlib-*), but these weren't necessary to fix the error.
• build-essential package - because make and a cc (gcc) are required
• lld because it was cited in the description but not the apt commands
• git because the toolchain requires it. A potential issue if you're building in containers or remote servers

Removed

• pop-launcher as a dependency, because it's actually a build target of this repo.

Testing

• Built cosmic in 24.04 and 25.10 containers using just sysext
• Tested the Cosmic build on Ubuntu 25.10. Worked on this PR on Cosmic!

Discussion

Package names are repeated up to 3 times in this section. I would argue that the repetition adds little value, I would prefer to deduplicate the entries if that's amenable.

Ideally I would also want to classify dependencies as build-time required, run-time required, or both. For example, I originally built the binaries in a container, and then Cosmic wouldn't run on the host OS until I installed the same dependencies on the host. I think that's a bit of a project though, and outside of this PR's scope.