fix: Correct CSV file path in update-lgsm command (#4917)
The distro CSV file was being fetched with an absolute path
instead of a repository-relative path, causing malformed URLs
like /master//home/user/lgsm/data/debian-12.csv
Changed fn_fetch_file_github first parameter from "${datadir}"
to "lgsm/data" to match the correct repository path structure.
Fixes #4856: ERROR: Downloading debian-12.csv on update-lgsm