feat: add appimage back to ci

[Green-Sky]

things still to do:

• make own alpine image with up-to-date toxcore!!
• find a solution for the go-appimage download. it currently always downloads the latest. (they only provide the latest !!)
• test non standalone deploys they are funny and dont work
• check if the .zsync update diff is properly generated (how?)
• ccache
• fix arch? in gh artifact
• add arm64 arch?

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11.02%. Comparing base (52d8c6d) to head (9a0b069).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
- Coverage   11.02%   11.02%   -0.01%     
==========================================
  Files         296      297       +1     
  Lines       21442    21450       +8     
==========================================
  Hits         2365     2365              
- Misses      19077    19085       +8     

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