ipn/ipnlocal: fix a case where ts_omit_ssh was still linking in x/crypto/ssh

And add a test.

Updates #12614

Change-Id: Icb1c77f5890def794a4938583725c1a0886b197d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/ssh_stub.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build ios || android || (!linux && !darwin && !freebsd && !openbsd && !plan9)
+//go:build ts_omit_ssh || ios || android || (!linux && !darwin && !freebsd && !openbsd && !plan9)
 
 package ipnlocal