refactor: set DOKKU_SYSTEM_USER and DOKKU_SYSTEM_GROUP at the top-level

plugins/common/properties.go

@@ -144,13 +144,6 @@ func setPermissions(path string, fileMode os.FileMode) (err error) {
 
 	systemGroup := os.Getenv("DOKKU_SYSTEM_GROUP")
 	systemUser := os.Getenv("DOKKU_SYSTEM_USER")
-	if systemGroup == "" {
-		systemGroup = "dokku"
-	}
-	if systemUser == "" {
-		systemUser = "dokku"
-	}
-
 	group, err := user.LookupGroup(systemGroup)
 	if err != nil {
 		return