cmd/tailscale: fix 'debug local-creds' hostname

Fixes #6446

Change-Id: I82f0a3dcf6aca25b7f67265533ee30a9d939d86f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f