`-overwrite=true` and `-with-passphrase=false` flags don't work for `principal fork`

[razvanm]

The -overwrite=true and -with-passphrase=false flags don't seem to work work for principal fork at the current head (9e2f983).

An easy way to demonstrate this issue:

$ rm -rf .v23
$ principal fork -overwrite=true -with-passphrase=false -for=72h .v23 xxx
Enter passphrase (entering nothing will store the principal key unencrypted):
$ principal fork -overwrite=true -with-passphrase=false -for=72h .v23 xxx
Enter passphrase (entering nothing will store the principal key unencrypted):
ERROR: failed to create principal: failed to open .v23/publickey.pem for writing: open .v23/publickey.pem: file exists
$

The similar flags work fine for principal create.