ci: allow the installation of greenboot rpm packages from compose

[mcattamoredhat]

This PR introduces changes to allow the installation of greenboot RPM packages from a given
RHEL compose, and extends bootc test coverage to RHEL 9.9 and RHEL 10.3.

ci: allow the installation of greenboot rpm packages from compose

When both DOWNLOAD_NODE and COMPOSE_ID environment variables are set, the qcow2,
anaconda-iso, and ostree CI test cases will fetch the greenboot RPMs directly from the RHEL
compose available at the DOWNLOAD_NODE URL, instead of building them locally with make rpm.
This makes local testing against specific RC or nightly composes simpler and more automated.

The RPM download logic has been extracted into a shared helper
(tests/common/download-compose-rpms.sh) to avoid duplication across scripts. The helper
fetches the packages index page once and reuses it for both RPM lookups.

ci: add RHEL 9.9 and RHEL 10.3 support to bootc test cases

Extends the bootc qcow2 and anaconda-iso test scripts to support RHEL 9.9 and RHEL 10.3
alongside the existing 9.8 and 10.2 variants. Each new distro gets its own DNF repo file
with a REPLACE_ME_HERE placeholder substituted at runtime from DOWNLOAD_NODE. CI jobs
rhel-9-9-bootc and rhel-10-3-bootc are added to greenboot-ci.yaml targeting the
corresponding nightly composes.

Note: The ostree test case is not updated yet as RHEL 9.9 support there is not ready.

[knecasov]

I added a few comments.

Have you considered extracting the shared parts of the code (for example, an RPM download logic) into a common helper, please? The same block is repeated across all three scripts.

[say-paul]

LGTM

[mcattamoredhat]

I added a few comments.

Have you considered extracting the shared parts of the code (for example, an RPM download logic) into a common helper, please? The same block is repeated across all three scripts.

Done!

[say-paul]

LGTM