Fix latest alma9 release breaking debug packages

On the morning of 27th November, Linux@CERN released a new update of the alma9-base image that introduced a breaking change to the way debug packages are created.

The extracting debug info from ... parts fail with gdb-add-index: Failed to find a useable GDB binary. I'll look into fixing this. I think the easiest solution is to first revert to a specific alma9 version.

To prevent these types of issues in the future, see #975