Revert "Use GCC9 for building Krita AppImages on the binary factory"
It looks like GCC9 makes the code depend on the newer version of libstdc++.so.6, which breaks compatibility with Ubuntu 18.04. If we embed libstdc++.so.6 into the AppImage, then openGL drivers for Radeon stop working (work fine for Intel though).
This reverts commit 2ea9cda1.