Uncommon Nix derivation attributes

• dis/allowedReferences specifies a list of dis/allowed dependencies for a derivation
• dis/allowedRequisites specifies a list of dis/allowed dependencies in the entire closure of a derivation (i.e., the derivation itself and its recursive dependencies)
• impureEnvVars specifies a list of environment variables to pass from the caller’s environment into the builder’s. This is only allowed in fixed-output derivations^ᛦ
• passAsFile specifies a list of attribute names that ought to be passed as files (say, a large string big to be passed in as a path at $bigPath in the builder)