m/t/installer: re-add a missing import

Change-Id: I8c881aa6f23de5bf985e7015e0e649eafa094b10
Reviewed-on: https://review.monogon.dev/c/monogon/+/504
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/test/installer/main.go b/metropolis/test/installer/main.go
index 0a5e5aa..dafa288 100644
--- a/metropolis/test/installer/main.go
+++ b/metropolis/test/installer/main.go
@@ -20,6 +20,7 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"