fix(libstore/build/derivation-goal): don't assert on partially valid outputs

[lovesegfault]

Motivation

Fixes: #14130 (maybe?)

Context

Claude-aided debugging:

1. A derivation has multiple outputs (e.g., out and dev)
2. One output (out) is already valid in the store
3. Another output (dev) is NOT valid
4. User requests to build the already-valid output (out)
5. Since not ALL outputs are valid, DerivationGoal cannot return early—it must proceed with the build
6. DerivationGoal delegates to DerivationBuildingGoal to rebuild the entire derivation
7. In registerOutputs() (derivation-builder.cc:1383-1388), already-valid outputs are skipped to avoid unnecessary re-registration
8. The wantedOutput (out) is skipped, so it's NOT added to builtOutputs
9. Back in DerivationGoal, the code tries to filter builtOutputs to only contain wantedOutput
10. Assertion fails because wantedOutput is not in builtOutputs

I'm still trying to write a reproducer to validate...

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