Set up AppImageUpdate

[TheAssassin]

The Popsicle AppImage works great. However, AppImageUpdate should be set up to allow users to easily update on new releases. All that needs to be done is:

• set the environment variable UPD_INFO before calling appimagetool (when using linuxdeploy with --output appimage, you have to set it before that) to gh-releases-zsync|pop-os|popsicle|latest|Popsicle_USB_Flasher-*.AppImage
• upload the now-created .zsync file along with the AppImage (typically, you just need to change the pattern from ...AppImage to ...AppImage*

For uploading, you may want to switch to https://github.com/TheAssassin/pyuploadtool/. Then, the build job can then be run on every commit to build a continuous release. pyuploadtool uploads the files to a prerelease called "continuous" when building commits and releases called like the tag when building tags.

[mmstick]

Would you be willing to submit these changes as a PR?

[TheAssassin]

Done. See #141. It's not 100% complete, but you can easily complete it.