Fix countme bucket calculation

[dmnks]

Instead of storing the epoch per-repo, use the machine-id(5) file's timestamp as the source of truth. Please see the commit for details (GH didn't auto-fill the description here when creating the PR for some reason).

Note: Only intended for F41+ (should be reverted in F40 and lower).

Related test PR: https://github.com/rpm-software-management/ci-dnf-stack/pull/1501

Fixes: #1611

[dmnks]

Please note the countme test now fails in the CI since it's not compatible with the new code, that's fixed by https://github.com/rpm-software-management/ci-dnf-stack/pull/1501.

[jan-kolarik]

/packit test --identifier dnf-tests