hostinfo: set Hostinfo.PackageType for mkctr container builds

Fixes tailscale/corp#21448

Change-Id: Id60fb5cd7d31ef94cdbb176141e034845a480a00
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f