treewide: replace error comparisons and assertions with errors.Is

Change-Id: Id2424eb155f2c6842c72c5fafd124d428ef901f2
Reviewed-on: https://review.monogon.dev/c/monogon/+/2994
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/metropolis/cli/metroctl/cmd_k8scredplugin.go b/metropolis/cli/metroctl/cmd_k8scredplugin.go
index 9b3226e..adc3fe8 100644
--- a/metropolis/cli/metroctl/cmd_k8scredplugin.go
+++ b/metropolis/cli/metroctl/cmd_k8scredplugin.go
@@ -4,6 +4,7 @@
 	"crypto/x509"
 	"encoding/json"
 	"encoding/pem"
+	"errors"
 	"log"
 	"os"
 
@@ -27,7 +28,7 @@
 
 func doK8sCredPlugin(cmd *cobra.Command, args []string) {
 	cert, key, err := core.GetOwnerCredentials(flags.configPath)
-	if err == core.NoCredentialsError {
+	if errors.Is(err, core.NoCredentialsError) {
 		log.Fatal("No credentials found on your machine")
 	}
 	if err != nil {