missing input deps is a println!, not fatal error

[colemickens]

Some flakes don't have nixpkgs, be a bit more accepting of them.

[lucperkins]

@colemickens The tool does currently enable you to supply a specific set of Nixpkgs keys via --nixpkgs-keys. I'd prefer to keep the current behavior.

[colemickens]

@lucperkins for context, this is for testing with DeterminateSystems/ci.

Not having this would imply needing another option for ci to teach it about what branches to check. I'd be curious for @grahamc's thoughts on that.

[colemickens]

For now, I can probably set NIX_FLAKE_CHECKER_NIXPKGS_KEYS for my parent workflow...

[lucperkins]

The issue is that there's always an expected set of Nixpkgs input keys. The default is ["nixpkgs"] but you can override that. With the behavior proposed in this PR, Flake Checker would simply successfully exit if none of the provided keys actually were Nixpkgs inputs, which would be surprising behavior, I think.