unify: modify logging to clarify whether `flake.lock` changed at all

[llakala]

Currently, whether we got no changes, or only unimportant inputs were changed, we get this message:

No important updates to flake.lock, so skipping rebuild

I'd prefer to have a different message in both instances. Something like:

No inputs have been changed since last execution

As opposed to:

Some inputs were updated, but they weren't in the list of inputs that trigger a rebuild/commit

Relies on #22. I could do this within there, or after it's merged.

[llakala]

#22 has since been merged, so I can do this anytime.