Motivation

The process that generates the updater channel files is broken in the CI

Change description

Move the renaming of the channel files (beta*.yml and latest*.yml into nightly*.yml and stable*.yml) inside the electron packager script.

Other information

Apparently, electron-builder doesn't allow custom channels, so the only channels available are alpha, beta and latest. Since we want to use different channel names (stable and nightly) we need to rename the channel files after the packaging of the app.

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• PR addresses a single concern.
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• Docs have been added / updated (for bug fixes / features)