remote-build: new command logic

[mr-cal]

What needs to get done

• Implement the new work area, where the WorkTree is not reused and removed after the build completes.
  • Each launchpad build has a unique build-id and WorkTree. In the legacy code, WorkTrees are stored by build-id in ~/.local/share/snapcraft/remote-build/<build-id>/.
  • In the new code, WorkTrees will be stored in the work area ~/.cache/snapcraft/remote-build/<build-id>/. When the build completes successfully, the WorkTree will be removed. WorkTrees are not reused.
• use new _generate_build_id() from remote-build: migrate build-id generation #4322
• linting and typing updates (e.g. use f-strings)
• move snapcraft business logic to snapcraft/commands/remote.py

Why it needs to get done

This allows a user to inspect the contents of the WorkTree if the build fails and minimizes disk usage.