wgengine/router: probe generally-unused "ip" command style lazily

This busybox fwmaskWorks check was added before we moved away from
using the "ip" command to using netlink directly.

So it's now just wasted work (and log spam on Gokrazy) to check the
"ip" command capabilities if we're never going to use it.

Do it lazily instead.

Updates #12277

Change-Id: I8ab9acf64f9c0d8240ce068cb9ec8c0f6b1ecee7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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