ci: verify opensource RPM install on AlmaLinux 9 in addition to 8

[esolitos]

What

• Run the RPM install check on both AlmaLinux 8 and 9 (matrix; powertools repo on el8, crb on el9).

Why

• EL9 RPMs are being added to the open-source Cloudsmith repository and should be install-verified like el8.

Additional info

• Keep as draft until the first EL9 RPMs are published — the el9 job fails until then
• Check name changes to verify-opensource-rpm-installable (8, powertools) / (9, crb); update any branch-protection rule keyed on the old name

How to test

• Re-run the workflow once EL9 RPMs exist; both legs must pass.