https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41706 attempted to
prevent `rsync` from including tmp directories when preparing backups
with `STRATEGY=copy`, but used an incorrect path that caused `rsync`
to continue to include these directories.

The `--exclude` argument was passed with the tmp directory name with a
leading slash, but this is not the correct format when no trailing slash
is used on the source path. `rsync` will `chdir` to the parent directory
in this scenario, e.g. /var/opt/gitlab/gitlab-rails/shared for pages,
treating that location as `/` in the `--exclude` path.

We need to include full path from the source directory to the tmp
directory in `--exclude`, which means adding the basename of
`app_files_dir`.

Current command:
  rsync -a --delete --exclude=lost+found --exclude=/@pages.tmp \
    /var/opt/gitlab/gitlab-rails/shared/pages /var/opt/gitlab/backups

Correct command:
  rsync -a --delete --exclude=lost+found --exclude=/pages/@pages.tmp \
    /var/opt/gitlab/gitlab-rails/shared/pages /var/opt/gitlab/backups