rpm: add signing to fluent-release package

[Watson1978]

This PR will add signing to fluent-release package due to fix following Package fluent-release-2025.9.29-1.el9.noarch.rpm is not signed message with sudo yum update.

$ sudo yum update -y
Fluentd Project                                 177 kB/s |  80 kB     00:00
Dependencies resolved.
================================================================================
 Package            Arch       Version              Repository             Size
================================================================================
Upgrading:
 epel-release       noarch     9-10.el9             epel                   19 k
 fluent-release     noarch     2025.9.29-1.el9      fluent-package-v6      12 k

Transaction Summary
================================================================================
Upgrade  2 Packages

Total download size: 31 k
Downloading Packages:
(1/2): epel-release-9-10.el9.noarch.rpm         298 kB/s |  19 kB     00:00
(2/2): fluent-release-2025.9.29-1.el9.noarch.rp  42 kB/s |  12 kB     00:00
--------------------------------------------------------------------------------
Total                                            19 kB/s |  31 kB     00:01
Extra Packages for Enterprise Linux 9 - x86_64  1.6 MB/s | 1.6 kB     00:00
Importing GPG key 0x3228467C:
 Userid     : "Fedora (epel9) <epel@fedoraproject.org>"
 Fingerprint: FF8A D134 4597 106E CE81 3B91 8A38 72BF 3228 467C
 From       : /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-9
Key imported successfully
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'yum clean packages'.
Package fluent-release-2025.9.29-1.el9.noarch.rpm is not signed

[daipom]

Hmm, it appears that this feature is intended to sign a specific version, but with this change, it will sign all fluent-release packages every time, which concerns me a bit.
From the code, it seems like we can already sign fluent-release packages just by running something like manage-fluent-repository {path} 2025.9.29 without changing the script.
So, for now, it might be better to tweak the release steps rather than the script.