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 <bradfitz@tailscale.com>
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