CI improvements

[primeos-work]

• Drop the checklist from the PR template and check if the commits are signed off: Automate the commit checks and move the checklist to CONTRIBUTING.md #110
• Fix caching: actions: Improve the CI caching and simplify the cargo CLI options #131
• Make the cargo-deny warnings more visible / fatal
• Avoid warnings (etc.)
• Stop ignoring cargo clippy warnings Lint with the beta toolchain as well (but only make the results with the MSRV binding): actions: Run optional clippy (linting) checks with the beta toolchain #111
• Analyze the security
• CLI/integration testing: Cli testing #43
• Nix build (sandboxed and reproducible)
• CD? (upload binary / build artifact)
• Enforce consistent formatting (rustfmt) via CI: Format the codebase via rustfmt #263
• Other improvements (TODO)