- Don't overwrite `nix-bitcoin-release.nix` on errors - Show a message to indicate whether `nix-bitcoin-release.nix` was updated - Don't start a shell when called noninteractively Also, update `usage.md` and reformat `shell.nix`.
- Don't overwrite `nix-bitcoin-release.nix` on errors - Show a message to indicate whether `nix-bitcoin-release.nix` was updated - Don't start a shell when called noninteractively Also, update `usage.md` and reformat `shell.nix`.