[CI] MacOS: Add a Gajim version parameter in CI mode and a release workflow

Goal

• Build a specific version of Gajim (wich pull omemo-dr and nbxmpp with pip using versions defined in the pyproject file) in CI.
• Add a Github workflow to run a release build each time a new tag is pushed
  • Upload the resulting dmg file in ./downloads/${gajim_minor_version}/Gajim-${gajim-version}-${arch}.dmg FTP directory

Information

• This PR also fix a mistake by using $GITHUB_OUTPUT instead of the deprecated set-output