use /etc/ccache.conf in buildroot

[nekopsykose]

currently we set CCACHE_ vars in env; this works (and with high priority, see order in https://ccache.dev/manual/latest.html#_configuration), but it's not guaranteed a given build respects the env. for instance, scons drops the entire env (except for instance PATH) when invoking the compiler, and so the actual CCACHE_DIR is lost; the invoked ccache will default to $HOME/.ccache in /tmp and then be deleted right after. (small loss since nothing uses scons, but annoying behaviour, and maybe this happens to more random builds)

it would be a lot neater (fuck env vars) if instead cbuild wrote the config to /etc/ccache.conf in the buildroot, which avoids env related issues and has no downsides i can think of. local ccache configuration still takes precedence, so this doesn't break anything for local user config in the cache_dir.