keybase fs cp -f [SOURCE] [DEST] — the force flag doesn't work anymore

> Note, the kbfs repo is mysteriously archived now, so ... I am filing this here.

It's been broken for a bit now, but if `somefile` already exists at the destination, it fails to copy even if `-f` is used:
```
[myusername@blahblahblah ~]$ keybase fs cp -f ./tmp/somefile /keybase/private/myusername/
▶ ERROR name exists (code 5101)
```

Start by reproducing the `keybase fs cp -f ./tmp/somefile /keybase/private/myusername/` command and trace the `keybase fs cp` entry point handling the force flag. Confirm the behavior with an existing destination; done means the command replaces the existing destination instead of returning the `name exists` error.

Written by the indexing model from the issue text.