Roll sr.ht build secrets
Update niv sources and allow building with custom nixpkgs collection
Allow empty content for commands that read from standard input
README clarifications and installation instructions for NixOS
Update manual with default comment input source for `update ticket` command
Make `update ticket` command take comment from stdin by default
Update niv sources
Factor Nix build config into beaker library
Add NixOS build manifest for testing
Add nix configs and niv sources
Add man page build instructions to README
Clarify behaviour of "get manifest" command in man page
Switch OpenBSD build manifest to always use latest version
Add "cancel job" command to CLI
Add --tags flag to "create job" command
Add cancel procedure to builds library
Some consistency tweaks for CLI flag handling and help text
Drop man page viewer test

This isn't robust enough for cases where man(1) is missing, or the
viewer doesn't include the text we're looking for, or whatever else.
Better no test than a one that throws false negatives, for now.
Add ticket create and update operations to CLI
Add support for ticket creation