client/tailscale: fix ExpandSNIName on non-default LocalClient

It was using a mix.

Found by @maisem.

Change-Id: Ieb79d78608474ac13c2f44e0f3d8997a5665eb13
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f