As you can see below I have to set the work-tree of a bare repo:
cd barerepo
git status
fatal: This operation must be run in a work tree
git --work-tree=/va
Note that the question and answers are from year 2012, but since git 2.5, even with a bare repository you can create separate worktrees with:
$ git worktree add /var/www/mywork/ master
$ git worktree add /var/www/workdev/ devel
See git-worktree(1).
It will not change core.worktree but create a worktrees directory in your git repository.
You may want to change the configuration option extensions.worktreeConfig to true if you don't want all the worktrees and the bare repository to share the same config.