Use built-in 'xz' post-processor

[arky]

Packer now offers built-in support for xz format archives. It is possible to archive without using external script and tool 'pixz' for this.

Here is example snippet for generating a archive which is directly flashable using etcher.

  post-processor "compress" {
    output  = "{{.BuildName}}.xz"
    compression_level   = 9
    keep_input_artifact = false
  }