ipn/ipnlocal: prevent attempting to run SSH on Synology for now

On DSM7 as a non-root user it'll run into problems.

And we haven't tested on DSM6, even though it might work, but I doubt
it.

Updates #3802
Updates tailscale/corp#5468

Change-Id: I75729042e4788f03f9eb82057482a44b319f04f3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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