gokrazy: use monorepo for gokrazy appliance builds (monogok)

This switches our gokrazy builds to use a new variant of cmd/gok called
opinionated about using monorepos: https://github.com/bradfitz/monogok

And with that, we can get rid of all the go.mod files and builddir forests
under gokrazy/**.

Updates #13038
Updates gokrazy/gokrazy#361

Change-Id: I9f18fbe59b8792286abc1e563d686ea9472c622d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

gokrazy/monogok

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+# This is a wrapper around monogok that sets the working directory.
+
+dir=$(dirname "${BASH_SOURCE[0]}")
+
+cd $dir
+$dir/../tool/go run github.com/bradfitz/monogok/cmd/monogok "$@"