Override some craft settings when building the centos 7 cache
Most notably, use a different cache key to make sure we don't accidentally try to reuse the cache build with the other ubuntu builder.
That said - I'm not really sure if this is required. I can see e.g.: https://files.kde.org/craft/master/Qt_5.15.1/linux/64/gcc/Release/ Which is the cache generated by the ubuntu builder. I believe with this patch, we'd instead replace Qt_5.15.1 with centos7? Is that OK? I'm surprised to not see a manual CacheVersion variable being set in the main master/CraftBinaryCache.ini, only the prebuilt/ and experimental/ versions do that?
But we definitely want to disable building xattr and instead use the system library. And since we use this to build app images, we also kde/frameworks/tier1/breeze-icons.useIconResource.