lib: Add optionalDrvAttr to conditionally set drv attributes.

This allows for adding new, conditionally set, derivation attributes
to an existing derivation without changing any output paths in the
case where the condition is not met.

doc/default.nix

@@ -25,6 +25,7 @@ let
       { name = "gvariant"; description = "GVariant formatted string serialization functions"; }
       { name = "customisation"; description = "Functions to customise (derivation-related) functions, derivatons, or attribute sets"; }
       { name = "meta"; description = "functions for derivation metadata"; }
+      { name = "derivations"; description = "miscellaneous derivation-specific functions"; }
     ];
   };