If this environment variable is set on the host, it's going to mess up
authentication in the sandbox. For example, if the host has:
KRB5CCNAME=KCM:
then the sandboxed process will try to use the host KCM socket, which is
not available in the sandboxed environment, rather than the gssproxy
socket that we want it to use. We need to unset it to ensure that
whatever configuration we ship in the runtime gets used instead. We have
switched the GNOME runtime to use an empty krb5.conf and it works as
long as we don't break it with this environment variable meant for the
host.
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