--input comes "too late" and cannot override default values about dagger

[aluzzardi]

I think this might be related to #19?

[aluzzardi]

This is actually related to #71.

Currently, the env exports base, input and output as JSON, only to be merged back as Cue in the end.

However, exporting to JSON is lossy, and the merge will fail.

For instance,

• base: lol: string | *"default lol" --> .JSON() --> {"lol":"default lol"}
• --input 'lol: "hello"' --> .JSON() --> {"lol":"hello"}

The merge between the two will end up failing.

I'm expanding the scope of #71 to fix this as well.