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CI: ensure build directory is clean first
The CI script should rm the build directory before starting a build to ensure it does not accidentally depend on old contents. CI does not automatically do that, which can be seen with the graphene subproject not always needing a new download.
But if I rm then the subproject will be re-downloaded on every build. For now it is small enough to be insignificant, and maybe even wanted, but maybe it could be cached too. How?