blender currently fails to build with:

    ./source/blender/imbuf/intern/oiio/openimageio_api.cpp:246: error: undefined reference to 'OpenImageIO_v2_3::TypeDesc::basesize() const'
    /usr/include/OpenImageIO/paramlist.h:107: error: undefined reference to 'OpenImageIO_v2_3::ParamValue::clear_value()'
    /usr/include/OpenImageIO/paramlist.h:107: error: undefined reference to 'OpenImageIO_v2_3::ParamValue::clear_value()'
    /usr/include/OpenImageIO/paramlist.h:107: error: undefined reference to 'OpenImageIO_v2_3::ParamValue::clear_value()'
    collect2: error: ld returned 1 exit status

This reverts commit f2e88e29.

https://bugs.debian.org/1009858

https://patchwork.freedesktop.org/patch/470224/

.gitlab-ci.yml: don't set schroot union-overlay-directory now that we mount a tmpfs to /var/lib/schroot/union/overlay

This reverts commit f5fa3b58.