[cipd] make creating instances from cipd installations work.

cipd/client/cipd/local/builder.go

Index: cipd/client/cipd/local/builder.go
diff --git a/cipd/client/cipd/local/builder.go b/cipd/client/cipd/local/builder.go
index ec0b3f0df53c61818aaa97559c05f42360286336..cbfb2c2224a93a9ec8b287e47359829a3d51be5f 100644
--- a/cipd/client/cipd/local/builder.go
+++ b/cipd/client/cipd/local/builder.go
@@ -55,11 +55,16 @@ func BuildInstance(ctx context.Context, opts BuildInstanceOptions) error {
 		return err
 	}
 
-	// Make sure no files are written to package service directory.
+	// Sanitize the Inputs.
 	for _, f := range opts.Input {
+		// Make sure no files are written to package service directory.
 		if strings.HasPrefix(f.Name(), packageServiceDir+"/") {
 			return fmt.Errorf("can't write to %s: %s", packageServiceDir, f.Name())
 		}
+		// Make sure no files are written to cipd's internal state directory.
+		if strings.HasPrefix(f.Name(), SiteServiceDir+"/") {
+			return fmt.Errorf("can't write to %s: %s", SiteServiceDir, f.Name())
+		}
 	}
 
 	// Generate the manifest file, add to the list of input files.