Feature request: Use single expression to build attributes

[dtzWill]

I ran into this due to the resulting memory usage problems that cause nox-review to scale badly (at least recently).

For some details and a bit of context, see: https://github.com/NixOS/nixpkgs/pull/32870#issuecomment-353173989

Related issue that would be fixed by fix proposed in my comment above: https://github.com/madjar/nox/issues/73

Thoughts? Objections? Once folks agree this is a "good idea" it should be straightforward to implement.

[veprbl]

But technically this is a bug in nix, not in nox?

[dtzWill]

That's fair, yes. I'd say sending via stdin (as in the PR) is a nox improvement since it allows running nix on input strings longer than max command-line length (platform limitation). But reworking how to express what to build (this issue) is working around a nix bug, yes.