util/cloudenv: detect Hetzner Cloud

Detect Hetzner via /sys/class/dmi/id/sys_vendor == "Hetzner" and wire
up Hetzner's public recursive DNS resolvers (185.12.64.1, 185.12.64.2)
for use as a cloud host resolver.

Fixes #20217

Change-Id: I24a4c51956adfdd5731f62c937e3c7a4a733ffc7
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