adjust for downstream branch name change

[tshepang]

Else we get a CI fail

fatal: couldn't find remote ref master
Traceback (most recent call last):
  File "/home/runner/work/fls/fls/./make.py", line 158, in <module>
    main(os.path.abspath(os.path.dirname(__file__)))
  File "/home/runner/work/fls/fls/./make.py", line 152, in main
    linkchecker = build_linkchecker(root)
                  ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/fls/fls/./make.py", line 90, in build_linkchecker
    git(["fetch", "--depth=1", "origin", "master"])
  File "/home/runner/work/fls/fls/./make.py", line 77, in git
    subprocess.run(["git", *args], cwd=repo, check=True)
  File "/usr/lib/python3.12/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['git', 'fetch', '--depth=1', 'origin', 'master']' returned non-zero exit status 128.