nixos-enter sets up /etc/resolv.conf as a bind mount from the host
system, so trying to activate a system that sets
`environment.etc."resolv.conf"` (e.g. with systemd-resolved enabled)
results in an unhelpful warning.
Skip linking /etc/resolv.conf if we're in a nixos-enter environment, as
determined by the IN_NIXOS_ENTER environment variable.
Make the warnings more helpful, indicating which file we failed to link.
Unlink temporary files in case of failure.
37e42d01a0