mantle: fix errors when building tests

```
[coreos-assembler]$ make mantle-check
cd mantle && ./test
Building tests...
?       github.com/coreos/coreos-assembler/mantle/auth  [no test files]
platform/metal.go:315:26: non-constant format string in call to fmt.Errorf
kola/harness.go:1309:14: non-constant format string in call to (*github.com/coreos/coreos-assembler/mantle/harness.H).Fatalf
?       github.com/coreos/coreos-assembler/mantle/cli   [no test files]
kola/tests/misc/network.go:172:12: non-constant format string in call to (*github.com/coreos/coreos-assembler/mantle/harness.H).Errorf
kola/tests/misc/network.go:668:12: non-constant format string in call to (*github.com/coreos/coreos-assembler/mantle/harness.H).Fatalf
kola/tests/misc/network.go:673:13: non-constant format string in call to (*github.com/coreos/coreos-assembler/mantle/harness.H).Fatalf
```

I asked for help from AI on this and it took care of things for me.

Assisted-By: <google/gemini-2.5-pro>

Author: dustymabe

mantle/kola/harness.go

@@ -1306,7 +1306,7 @@ ExecStart=%s
 						plog.Errorf("failed to get terminal via ssh: %v", err)
 					}
 				}
-				c.Fatalf(errors.Wrapf(err, "kolet failed: %s", stderr).Error())
+				c.Fatalf("kolet failed: %s: %v", stderr, err)
 			}
 		},
 

mantle/kola/tests/misc/network.go

@@ -169,7 +169,7 @@ func NetworkListeners(c cluster.TestCluster) {
 		return checkListeners(c, expectedListeners)
 	}
 	if err := util.Retry(3, 5*time.Second, checkList); err != nil {
-		c.Errorf(err.Error())
+		c.Errorf("%v", err)
 	}
 }
 
@@ -665,12 +665,12 @@ func getConnectionIpv4Addresses(c cluster.TestCluster, m platform.Machine, conne
 func checkExpectedMACs(c cluster.TestCluster, m platform.Machine, expectedMacsList []string) {
 	macConnectionMap, err := getMacConnectionMap(c, m)
 	if err != nil {
-		c.Fatalf(fmt.Sprintf("failed to get macConnectionMap: %v", err))
+		c.Fatalf("failed to get macConnectionMap: %v", err)
 	}
 
 	for _, expectedMac := range expectedMacsList {
 		if _, exists := macConnectionMap[expectedMac]; !exists {
-			c.Fatalf(fmt.Sprintf("expected Mac %s does not appear in macConnectionMap %v", expectedMac, macConnectionMap))
+			c.Fatalf("expected Mac %s does not appear in macConnectionMap %v", expectedMac, macConnectionMap)
 		}
 	}
 }

mantle/platform/metal.go

@@ -312,7 +312,7 @@ func (inst *Install) setup(kern *kernelSetup) (*installerRun, error) {
 		pxe.tftpipaddr = "10.0.2.2"
 		pxe.bootindex = "1"
 	default:
-		return nil, fmt.Errorf("Unsupported arch %s" + coreosarch.CurrentRpmArch())
+		return nil, fmt.Errorf("Unsupported arch %s", coreosarch.CurrentRpmArch())
 	}
 
 	mux := http.NewServeMux()