[MU3] Partial fix #279707: First steps towards automatic updates on Linux

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Partially resolves: https://musescore.org/en/node/279707

Creates a zsync delta update file for AppImages but doesn't yet upload it to a server as this has to be done by @igorkorsukov or another member of the in-house team.

For now, updates require an external tool like AppImageUpdate or AppImageLauncher, but if it works as expected then we can bundle a tool within MuseScore like we do with Sparkle on macOS.

See https://github.com/AppImage/AppImageSpec/blob/master/draft.md#update-information

Next steps to enable automatic updates...

Stable releases

Simply upload the zsync file to GitHub releases alongside the AppImage. The autoupdate software understands GitHub and will query the GitHub API to find the latest stable version.

Nightly builds

The autoupdate software doesn't know anything about OSUOSL, so the AppImage and zsync files MUST be made available at the following exact URLs:

• https://ftp.osuosl.org/pub/musescore-nightlies/linux/3x/nightly/MuseScoreNightly-latest-x86_64.AppImage
• https://ftp.osuosl.org/pub/musescore-nightlies/linux/3x/nightly/MuseScoreNightly-latest-x86_64.AppImage.zsync

Here "latest" is a fixed string, NOT a variable!!!

Whenever a new nightly build is uploaded on the 3.x branch, it must replace the old one at that same URL. Of course we want to keep historic nightlies available, so you should upload nightlies to version-dependent URLs like now and then make a symlink / redirect / physical copy of the latest one at the above URL. See PR #4757 for a suggestion of how to do this.

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