"The App is invalid." error when trying to install a flutter app. Create an app making tutorial for apps to install using AltStore.

With Impactor I used Codemagic to build flutter apps "on windows" with a free account and it worked great. Now if the .ipa would not even be revoked anymore, this would be my ideal setup.
I basically follow this tutorial (e.g. build the app with codemagic and compress it into the Payload folder) and then try to sideload my .ipa to AltStore. However I then sadly get the error "The App is invalid.". I read a bit about how UTM handles building and I think I do need to fake sign my app first, however I did not find out how to do that. So providing some documentation on how to build an app, so that AltStore can install it would be great. Maybe also provide a Github Actions example?

Sorry for that kind of weird request.

The .ipa file can be found here for reference.

Start with the linked Flutter-on-Windows tutorial and inspect the referenced empty.ipa to reproduce the AltStore "The App is invalid" failure. Document the build and signing steps needed for an AltStore-installable app, with a GitHub Actions example if supported, and verify the resulting instructions by installing the app.