AppImage building fails due to missing architectures

[JulianGro]

appimagetool, continuous build (commit 8bbf694), build <local dev build> built on 2020-12-31 11:48:33 UTC
More than one architectures were found of the AppDir source directory "/home/juliangro/Overte/Overte.AppImage"
A valid architecture with the ARCH environmental variable should be provided
e.g. ARCH=x86_64 appimagetool ...

https://bin.linux.pizza/?44582ca2954188a3#Aqcb5zuzt13nemuKfsotDyV47f2vMEGsh7Uyhzpg8JMw

[JulianGro]

Adding ARCH=x86_64 before the overte-builder command, works around this issue.