This is similar to running nixos-rebuild boot, just as nixops deploy --test is similar to running nixos-rebuild test. The flag creates a
new config and sets it as the default to boot into, but doesn't switch
to it right away.

%s has no special meaning to echo, so the ratchet script was printing Checking base branch at %s, then PR at %s...\n origin/master d2cb660583ce356af51d9978329c0e77ad284620.

Since we already have --test, I figured "why not go for all three major uses?" (the implicit switch, and the explicit --test and now --boot).

Sometimes I have a config I want to switch into a boot time (because it may be destructive to switch mid-session), so this is the perfect use for it. For example: recently, I was messing with udev rules. Setting services.udev.extraRules and switching into this configuration rendered my USB devices (mouse, keyboard) unresponsive until I rebooted. Now, in the future, I know that this is potentially game-over, so I want the change to be available at boot and not switch into it right away.

Let me know if I need to do anything else!

P.S. One thing I noticed when running the various CI scripts is that ci/check-tests.sh failed to check coverage with the error: nose.plugins.cover: ERROR: Coverage not available: unable to import coverage module. Maybe it's fine (and you just let CI handle it), but it might be nice to have a nix-shell shebang or have pkgs.python3.pkgs.nose in the shell's buildInputs.

EDIT: I imagine ratchet is failing because the new boot argument is also of type Any (due to def deploy(self, **kwargs: Any) -> None:). If not (and there's a way I can fix it), let me know.