Special derivation attributes

[infinisil]

Let's cover some special derivation attributes like:

• exportReferencesGraph (maybe useful for Nar Derivation  #57?)
• impureEnvVars: Already covered somewhat in Nix Hour 64
• outputChecks.
• (maybe __ignoreNulls?)

We will focus on this topic this week at Thursday 16:00 CEST! If anybody has any extra questions about this topic, either:

• Ask them in this issue in advance with a comment
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[infinisil]

@Ericson2314 pointed me to NixOS/nix#10760 which relates to this :D