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_takeownership.go b/metropolis/cli/metroctl/cmd_takeownership.go
index dd4bfb4..adad3cf 100644
--- a/metropolis/cli/metroctl/cmd_takeownership.go
+++ b/metropolis/cli/metroctl/cmd_takeownership.go
@@ -2,6 +2,7 @@
 
 import (
 	"context"
+	"errors"
 	"log"
 	"os"
 	"os/exec"
@@ -46,7 +47,7 @@
 	// Retrieve the cluster owner's private key, and use it to construct
 	// ephemeral credentials. Then, dial the cluster.
 	opk, err := core.GetOwnerKey(flags.configPath)
-	if err == core.NoCredentialsError {
+	if errors.Is(err, core.NoCredentialsError) {
 		log.Fatalf("Owner key does not exist. takeownership needs to be executed on the same system that has previously installed the cluster using metroctl install.")
 	}
 	if err != nil {