ipn/ipnlocal: add missing place where we set the SSH atomic
This fixes the "tailscale up --authkey=... --ssh" path (or any "up"
path that used Start instead of EditPrefs) which wasn't setting the
bit.
Updates #3802
Change-Id: Ifca532ec58296fedcedb5582312dfee884367ed7
Signed-off-by: Brad Fitzpatrick <[email protected]>