DerivationBuilderParams have reference to BasicDerivation

Have one to that instead of one to `Derivation`. `DerivationBuilder`
doesn't need `inputDrvs`, so `BasicDerivation` suffices.

(In fact, it doesn't need `inputSrcs` either, but we don't yet hve a
type to exclude that.)

src/libstore/include/nix/store/build/derivation-builder.hh

@@ -62,7 +62,7 @@ struct DerivationBuilderParams
     /**
      * The derivation stored at drvPath.
      */
-    const Derivation & drv;
+    const BasicDerivation & drv;
 
     /**
      * The derivation options of `drv`.