net/porttrack: change magic listen address format for Go 1.26

Go 1.26's url.Parser is stricter and made our tests elsewhere fail
with this scheme because when these listen addresses get shoved
into a URL, it can't parse back out.

I verified this makes tests elsewhere pass with Go 1.26.

Updates #18682

Change-Id: I04dd3cee591aa85a9417a0bbae2b6f699d8302fa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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