Automate upload of phar to release for self-update

Currently the self-update command returns

  [Exception]
  API error - no release found at GitHub repository extdn/installer-m2

consider switching to https://github.com/humbug/box

Works now with a manually added phar file to the tagged release. Consider https://github.com/humbug/phar-updater

Automate the upload of the phar file.