ccache fails to work: /sbuild-nonexistent/.ccache/tmp: Permission denied

It seems you have the ccache binary available in the build environment. That is good! However it seems to be configured to save the cache in the path "/sbuild-nonexistent/.ccache/". This is probably on the default setting, where the cache path = $HOME/.ccache.

You should set it something where there are write permissions. Either configure /etc/ccache.conf or set env CCACHE_DIR to something. See 'man ccache' for details.

Alternatively remove the ccache binary from the system, or any symlinks from the C compailers pointing to the ccache wrapper.

Example of failure: https://auto.debian.net/api/jobs/3355/log.txt