treewide: replace error assertions with errors.As

Change-Id: I369cc1dd8f745203f6f24093049d60d971acdf11
Reviewed-on: https://review.monogon.dev/c/monogon/+/3038
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/build/savestdout/savestdout.go b/build/savestdout/savestdout.go
index a0fb709..e8c8e75 100644
--- a/build/savestdout/savestdout.go
+++ b/build/savestdout/savestdout.go
@@ -17,6 +17,7 @@
 package main
 
 import (
+	"errors"
 	"log"
 	"os"
 	"os/exec"
@@ -43,7 +44,8 @@
 		return
 	}
 
-	if e, ok := err.(*exec.ExitError); ok {
+	var e *exec.ExitError
+	if errors.As(err, &e) {
 		os.Exit(e.ExitCode())
 	}