core: Always make AppDataDir for InstanceID (#5976)

caddy.go

@@ -825,13 +825,18 @@ func ParseDuration(s string) (time.Duration, error) {
 // regardless of storage configuration, since each instance is intended to
 // have its own unique ID.
 func InstanceID() (uuid.UUID, error) {
-	uuidFilePath := filepath.Join(AppDataDir(), "instance.uuid")
+	appDataDir := AppDataDir()
+	uuidFilePath := filepath.Join(appDataDir, "instance.uuid")
 	uuidFileBytes, err := os.ReadFile(uuidFilePath)
 	if os.IsNotExist(err) {
 		uuid, err := uuid.NewRandom()
 		if err != nil {
 			return uuid, err
 		}
+		err = os.MkdirAll(appDataDir, 0o600)
+		if err != nil {
+			return uuid, err
+		}
 		err = os.WriteFile(uuidFilePath, []byte(uuid.String()), 0o600)
 		return uuid, err
 	} else if err != nil {