Filter out non-directories up front

This way we error when given a path to a dir which contains files but
doesn't contain directories.

tools/key-verification/main.go

@@ -31,26 +31,30 @@ func main() {
 	if pathToKeys == "" {
 		pathToKeys = defaultKeysDir
 	}
-	devices, err := os.ReadDir(pathToKeys)
+	entries, err := os.ReadDir(pathToKeys)
 	if err != nil {
 		fmt.Fprintln(os.Stderr, "Error reading directory:", err)
 		os.Exit(1)
 	}
+	var devices []os.DirEntry
+	for _, device := range entries {
+		if device.IsDir() {
+			devices = append(devices, device)
+		}
+	}
 	if len(devices) == 0 {
 		fmt.Fprintln(os.Stderr, "No devices found in the directory:", pathToKeys)
 		os.Exit(1)
 	}
 	for _, device := range devices {
-		if device.IsDir() {
-			serial := device.Name()
-			path := filepath.Join(pathToKeys, serial)
-			err := verifyDeviceAttestation(serial, path)
-			if err != nil {
-				fmt.Fprintln(os.Stderr, "Error verifying device attestation:", err)
-				continue
-			}
-			fmt.Println("Device attestation verified for:", serial)
+		serial := device.Name()
+		path := filepath.Join(pathToKeys, serial)
+		err := verifyDeviceAttestation(serial, path)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "Error verifying device attestation:", err)
+			continue
 		}
+		fmt.Println("Device attestation verified for:", serial)
 	}
 }