Erik Arvstedt 77af2e4538
makeShell: improve update-nix-bitcoin
- 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`.
2021-09-14 19:56:35 +02:00
..
2021-09-14 19:56:35 +02:00