stackeval: Actually use the component block "inputs" argument

Due to some carelessness in earlier refactoring, the components block
evaluation was evaluating the "inputs" expression but not actually saving
its result, causing all components to get planned with all their input
variables set to default values (or failing, if there were any required).

internal/stacks/stackruntime/internal/stackeval/component_instance.go

@@ -85,6 +85,7 @@ func (c *ComponentInstance) CheckInputVariableValues(ctx context.Context, phase
 		}
 		expr = result.Expression
 		hclCtx = result.EvalContext
+		v = result.Value
 	}
 
 	if defs != nil {