instantiated: expose stderr output

instantiated.go

@@ -11,10 +11,6 @@ import (
 	"strings"
 )
 
-var (
-	Stdout io.Writer = nil
-)
-
 const (
 	instantiatedPrefix    = "instantiated '"
 	instantiatedSeparator = " -> "
@@ -74,6 +70,9 @@ func DecodeInstantiated(stderr io.Reader) ([]string, error) {
 
 		v := scanner.Text()
 		if !strings.HasPrefix(v, instantiatedPrefix) {
+			if Stderr != nil {
+				_, _ = Stderr.Write([]byte(v + "\n"))
+			}
 			continue
 		}
 		f := strings.SplitN(v, instantiatedSeparator, instantiatedFields)

io.go

@@ -0,0 +1,8 @@
+package nixbuild
+
+import "io"
+
+var (
+	Stdout io.Writer = nil
+	Stderr io.Writer = nil
+)