feat: set systemd inhibitor during sensative operations

[jcpunk]

There are a handful of times when an unexpected shutdown or reboot would leave the system in an incomplete state.

It would be helpful if bootc set systemd inhibitors during those windows to ensure the system is not kicked during those operations.

In practice systemd 257 is probably the lower bound for this to work as expected:

systemd-logind now always obeys block inhibitor locks, where previously it ignored locks taken by the caller or when the caller was root

[cgwalters]

We should be safe to forced reboots; if we're not it's a bug. We just fixed a major instance of that in https://github.com/ostreedev/ostree/pull/3511