DerivationBuilderImpl::unprepareBuild Just throw error

Aftet the previous simplifications, there is no reason to catch the
error and immediately return it with a `std::variant` --- just let the
caller catch it instead.

src/libstore/unix/build/linux-derivation-builder.cc

@@ -659,7 +659,7 @@ struct ChrootLinuxDerivationBuilder : ChrootDerivationBuilder, LinuxDerivationBu
             throw SysError("setuid failed");
     }
 
-    std::variant<BuildError, SingleDrvOutputs> unprepareBuild() override
+    SingleDrvOutputs unprepareBuild() override
     {
         sandboxMountNamespace = -1;
         sandboxUserNamespace = -1;