master

workflow reworked

- Replace unmaintained actions which used deprecated functions (Node.js 12,
  "set-output"), they may stop working soon:
  - https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/
  - https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

- Remove all hardcoded version numbers:
  To trigger a release, push a tag that starts with 'v', e.g. 'v1.1'.
  The release and filenames will be named accordingly (with the 'v'
  stripped from filenames, e.g. 'gx_plugins_1.1_src.atr.bz2')

- Disable 'Latest' tag
  Normal pushes (without a tag) will create pre-releases named after its
  branch, e.g. 'master'. 'Latest' only pointed to the latest workflow-run,
  and could even refer to older codebases.

- This code is heavily based on the workflow file from the Aether project
  /~https://github.com/Dougal-s/Aether/blob/master/.github/workflows/build.yml