Fix tabs with spaces in build.yml

[5310]

I too would really like a Flatpak now that FTB is off of Prism :pray:

Cleaned up the tabs from the newly edited Flatpak action on build.yml. It parses now, and the flatpak itself builds on my fork. Unfortunately every other build target fails due to various reasons for me :disappointed:

Hope this makes an official PollyMC Flatpak release channel go smoother :crossed_fingers:

[fn2006]

Thanks!