Fix struct update syntax deprecation warning elixir 1.19.0-dev

[nwjlyons]

Fixes

     warning: the struct update syntax is deprecated:

     %NimbleOptions.ValidationError{error | keys_path: path ++ error.keys_path}

     Instead, prefer to pattern matching on structs when the variable is first defined and use the regular map update syntax instead:

     %{error | keys_path: path ++ error.keys_path}

     │
 520 │         {:error, %ValidationError{error | keys_path: path ++ error.keys_path}}
     │                                  ~
     │
     └─ lib/nimble_options.ex:520:34

[josevalim]

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