[libstore]: Fix a heap-use-after-free bug

[GrahamDennis]

Motivation

Under high build load, we were seeing the nix daemon segfault around here. Further investigation suggested that this was a heap-use-after-free issue where the initialOutputs field referenced data in an activation frame that had since gone out of scope.

I believe the issue is caused by going through this while loop 3+ times:

1. The DerivationBuilder is constructed taking a reference to initialOutputs. The build doesn't complete but hits this continue statement here. Importantly, the DerivationBuilder is stored in the DerivationBuildingGoal
2. The number of current builds is greater or equal to the maximum number of builds and we call co_return tryToBuild() here. Importantly, I believe co_return of this form is implemented like a tail call, so the current tryToBuild activation frame is destroyed and hence initialOutputs referenced by the builder is destroyed too.
3. Inside builder->startBuild() triggers the heap-use-after-free issue.

There is at least one other way this could be fixed: by deleting this line and instead performing the build with another iteration of the while loop. It seems to me that a less fragile solution is to ensure that the builder doesn't take a reference to any variables in the current activation frame.

Context

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