No Story: Change CASA matrix link generation to only use pathlib

This fixes a bug where the matrix was failing to find any "pipeline" directories in CASA releases, so it wasn't generating any new links. It wasn't apparent this was happening until we noticed RHEL8 releases weren't being symlinked automatically.

This also changes link generation to only use pathlib for consistency. The original code came from the archive and used a combination of os and pathlib paths and got mega confusing when trying to debug this issue.

This was tested by copy-pasting most of the function in the interpreter on dev/test to generate the RHEL8 symlinks so PO testing can continue.