[leapp] collect /etc/leapp/files/dnf.conf

[pmoravec]

That file overrides default dnf.conf during leapp (pre)upgrade.

Closes: #4023

---

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/sosreport-sos-4023
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.