gokrazy, clientupdate: add start of Gokrazy auto-updates, tests

This adds support for Gokrazy GAF (Gokrazy Archive Format) zip
auto-updates, starting to wire up Tailscale's clientupdate mechanism
to Gokrazy's update mechanism.

Currently there's just a CLI command to update from a GAF URL,
with an --unsigned flag for use in a new natlab vmtest.

Next step would be publishing unstable track GAF files on
pkgs.tailscale.com, with detached signatures, and then making the
clientupdate mechanism also download those and check signatures.

Updates #20002

Change-Id: Ib03c56f17a57f8a4638398ef83549dac4813323d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.mod

@@ -17,7 +17,7 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/ssm v1.44.7
 	github.com/axiomhq/hyperloglog v0.0.0-20240319100328-84253e514e02
 	github.com/bradfitz/go-tool-cache v0.0.0-20260216153636-9e5201344fe5
-	github.com/bradfitz/monogok v0.0.0-20260429173803-229ef7981a6b
+	github.com/bradfitz/monogok v0.0.0-20260604043651-77f55eaefb19
 	github.com/bramvdbogaerde/go-scp v1.4.0
 	github.com/chromedp/cdproto v0.0.0-20260321001828-e3e3800016bc
 	github.com/chromedp/chromedp v0.15.1
@@ -103,7 +103,7 @@ require (
 	github.com/tailscale/netlink v1.1.1-0.20240822203006-4d49adab4de7
 	github.com/tailscale/peercred v0.0.0-20250107143737-35a0c7bd7edc
 	github.com/tailscale/setec v0.0.0-20251203133219-2ab774e4129a
-	github.com/tailscale/ts-gokrazy v0.0.0-20260429180033-fe741c6deb44
+	github.com/tailscale/ts-gokrazy v0.0.0-20260604151927-fc3a567bcf75
 	github.com/tailscale/web-client-prebuilt v0.0.0-20250124233751-d4cd19a26976
 	github.com/tailscale/wf v0.0.0-20240214030419-6fbb0a674ee6
 	github.com/tailscale/wireguard-go v0.0.0-20260527010701-b48af7099cad