Fix generation of update channel files in CI

[AlbyIanna]

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.

Reviewer checklist

• PR addresses a single concern.
• The PR has no duplicates (please search among the Pull Requests before creating one)
• PR title and description are properly filled.
• Docs have been added / updated (for bug fixes / features)