[cleaner] Use "hostname -f" in HostnamePrepper

[pmoravec]

HostnamePrepper should always be fed by FQDN to ensure domain names are recognized properly.

This is important esp. when:

• /etc/hosts is empty
• "hostname" contains shortname only
• "hostname -f" contains FQDN

Resolves: #4022
Closes: #4026

---

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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-4026
• And now you can install the packages.

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