Move the appliance/AMI build logic into tailscale.com/gokrazy/build so
Go callers (e.g. flash-appliance) can call it directly instead of
driving build.go over --json. build.go is now a thin flag wrapper; flags
and --json output are unchanged. Package-level state becomes a Builder
with an exported Config, ctx-first steps, and a Build one-shot.
Updates #1866
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
