ipnauth: set isUnixSock in the ts_omit_unixsocketidentity GetConnIdentity

The ts_omit_unixsocketidentity variant of GetConnIdentity never
performs the *net.UnixConn type assertion, so ConnIdentity.isUnixSock
stays false. ipnserver's Permissions only grants local API access to
unix socket connections, so with this build tag every local API request
is denied read and write access ("status access denied") and the CLI
cannot talk to the daemon at all in --extra-small/--min builds.

Mirror the type assertion from the peercred variant so the omitted
identity build behaves as intended (everyone is an admin when unix
socket identities are compiled out).

Signed-off-by: loowr <loowr@proton.me>