Allow substitutes for builds that have preferLocalBuild set

Not substituting builds with "preferLocalBuild = true" was a bad idea,
because it didn't take the cost of dependencies into account. For
instance, if we can't substitute a fetchgit call, then we have to
download/build git and all its dependencies.

Partially reverts 5558652709f27e8a887580b77b93c705659d7a4b and adds a
new derivation attribute "allowSubstitutes" to specify whether a
derivation may be substituted.

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