net/dns: add debug envknob to enable dual stack MagicDNS

Updates #15404

Change-Id: Ic754cc54113b1660b7071b40babb9d3c0e25b2e1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f