I'm not sure about the exact conditions for reproducing this issue, but
whenever I build Zed locally and have it open a single-file worktree on
launch, the rust-analyzer language server fails to start up because Zed
attempts to run `rust-analyzer --help` on a path that is not a
directory. This fixes that by running the command on the parent path in
the case of a single-file worktree.

Release Notes:

- Fixed rust-analyzer startup issue in single-file worktrees