Fix AppImage's architecture naming

The dependency jobs are failing because AppImageKit has its own custom
naming. I had added the case arm64 (dpkg) == aarch64 (AppImageKit),
but I missed that amd64 == x86_64.

The rest are OK or are unsupported by AppImageKit.

Please retry the dependency jobs after this commit.

(cherry picked from commit b379c564)

CCMAIL: kimageshop@kde.org