Features!

• add --system to build on other operating systems/architectures
• respect meta.broken in review shells
• read github tokens from gh client
• fix some missing logs for failed builds
• use buildEnv for mass-rebuilds to work around environment variable limits
• only enable build sandbox on Linux
• fix nix-shell when reviewing multiple PRs
• add --run flag to run commands instead of an interactive review shell
• write reports also as json