[nix-cache] Prevent checking cache twice per package

[mikeland73]

Summary

Fixes bug described here https://github.com/jetify-com/devbox/pull/2054

A better (but a bit more involved) solution is to parallelize by output. Currently we parallelize by meta-output (which includes __default_output__). This has two downsides:

• __default_output__ can be multiple outputs, in which case we don't parallelize them at all.
• We still do redundant work when __default_output__ is multiple outputs. (once as __default_output__ and once for each individual outout that forms part of defaults).

How was it tested?

[mikeland73]

I don't understand why fetchNarInfoStatus ended up calling twice. Did we end up getting different keys for the same installable?

yeah, we call it once per output and once for __default_output__.

So for a given package with a single (default) output out, we call it twice, once cached under key with out and once with key __default_output__