Consider updating countme logic to match dnf countme changes (using /etc/machine-id as system epoch)

[travier]

Describe the bug

The implementation of the count me logic in dnf was "per-repo" and thus was resetting the system time on every major Fedora update: rpm-software-management/libdnf#1611

The implementation in rpm-ostree does not suffer from this issue as it stores this information in a file independent of any repo: https://github.com/coreos/rpm-ostree/blob/main/rust/src/countme/cookie.rs#L11

We may or may not want to change our implementation to match the new one in dnf, which uses the /etc/machine-id file modification timestamp as the epoch for the system.

See: rpm-software-management/libdnf#1662

We can also keep things as is as this is not a problem for us right now and the current implementation works well enough.

Reproduction steps

N/A

Expected behavior

We use the same logic as dnf for our count me logic.

Actual behavior

We use a similar but not exactly the same logic.

System details

N/A

Additional information

No response