Bind mount /etc from tools tree into relaxed sandbox

Bind mount /etc from tools tree into relaxed sandbox

Config from /etc often references stuff in /usr. Two examples I've
encountered are shell config from /etc/profile and dnf5 blowing up
when there's plugin configuration in /etc without the corresponding
plugin being installed.

To work around such issues, let's use /etc from the tools tree in the
relaxed sandbox instead of /etc from the host. This also saves the user
from having to create directories in their host's /etc to be able to use
mkosi sandbox.