Some upstream systemd units are conditionally installed into the systemd
output, so we must make sure the feature that enables their installation
is enabled on our side prior to trying to use them.
tpm2.target was functionally useless without these services and this
generator. When systemd-cryptsetup-generator creates
systemd-cryptsetup@.service units, they are ordered after
systemd-tpm2-setup-early.service, not tpm2.target. These services are
themselves ordered after tpm2.target.
Note: The systemd-tpm2-setup(-early) services will serve no *function*
under a normal NixOS system at the moment. Because of their
ConditionSecurity=measured-uki, they will always be skipped, unless
you are building an appliance with the system.build.uki feature. Thus,
these are enabled solely for their systemd unit ordering properties.